For ROW_FORMAT=COMPACT and ROW_FORMAT=DYNAMIC records that contain
fields that have been inserted after columns were instantly added
(the record is tagged REC_STATUS_COLUMNS_ADDED), encode the
number of added columns - 1 in the record header.

rec_get_n_add_field_len): Determine the encoded size of the added fields.

rec_get_n_add_field(): Determine the number of added fields.

rec_set_n_add_field(): Set the number of added fields.

dict_col_t::is_nullable(): Determine if the column can be set NULL.

dict_col_t::is_nullable(): Determine if the column is virtual.

dict_col_t::is_instant(): Determine if the column has been instantly added.

dict_col_t::instant_value(): Get the default value of an instantly-added
column.

dict_index_t::get_n_nullable(n_prefix): Determine the number of nullable
index fields in the prefix.

dict_index_t::instant_field_value(): Get the default value of a
clustered index field of an instantly-added column.

For ROW_FORMAT=COMPRESSED tables or the change buffer tree,
the FIL_PAGE_TYPE_INSTANT is not allowed. It is allowed for
other types of user tables.

btr_search_move_or_delete_hash_entries(): Remove third parameter.
We will already have buf_block_t::index. In some callers, skip
the call if the index is known to be the spatial index, because
there is no adaptive hash index for spatial indexes.

Also, remove useless check for dict_table_is_locking_disabled()
for ROW_FORMAT=COMPRESSED tables. There will never be temporary
ROW_FORMAT=COMPRESSED tables in InnoDB.

btr_cur_search_to_nth_level(): Do not attempt to use or
update the adaptive hash index for MIN_REC_FLAG records
(either the 'default row' or node pointers).

btr_cur_optimistic_insert(), btr_cur_pessimistic_insert(),
btr_cur_optimistic_update(), btr_cur_pessimistic_update():
Do not attempt to update the adaptive hash index for
the 'default row' record.

btr_cur_update_in_place(): Assert that the 'default row' record
is not being updated so that the ordering fields (PRIMARY KEY)
would change.

btr_cur_optimistic_delete_func(), btr_cur_pessimistic_delete():
For now, assert that the 'default row' record will not be deleted.
(FIXME: It can and must be deleted in rollback.)

The ROW_FORMAT=COMPRESSED page format cannot be changed easily.
The schema is pretty much hard-coded.

btr_pcur_store_position() and btr_pcur_restore_position() will not
save the extra header associated with "instanly added" columns,
because the stored and restored prefixes only ever include key
columns, which cannot be added instantly.

For ROW_FORMAT=DYNAMIC and ROW_FORMAT=COMPACT records, we have to
identify whether the record contains more columns than when the
clustered index tree was originally created. In this case, the
number of fields
(FIXME: number_of_added_fields-1, not the total number of fields)
needs to be written to the record header.

rec_comp_status_t: An enum of the status bit values.

rec_leaf_format: An enum that replaces the bool parameter of
rec_init_offsets_comp_ordinary().

rec_get_converted_size_comp(): Do not support infimum,supremum.
They are never supposed to be stored in dtuple_t, as page creation
nowadays uses a lower-level method for inserting them.

innobase_add_instant_try(): Initialize the dtuple_t::info_bits
appropriately to REC_INFO_MIN_REC_FLAG | REC_STATUS_COLUMNS_ADDED
so that it can be inserted into a ROW_FORMAT!=REDUNDANT table.

row_merge_read_clustered_index(), IndexPurge::open():
Before testing for the 'default row' pseudo-record, advance
the cursor from the infimum pseudo-record, and then move back
if needed.

             idle_write_transaction_timeout

This is a preparatory step for MDEV-13864.
It does not change behavior in any ways. It simply splits methods into smaller peaces.
The intent of this separate patch is to make more readable the main patch for
MDEV-13864 (which will actually move the predicant to args[0]).

1. Splitting fix_length_and_dec() into smaller pieces, adding:
  - bool aggregate_then_and_else_arguments(THD *thd);
  - bool aggregate_switch_and_when_arguments(THD *thd);

2. Splitting find_item() into smaller pieces, adding:
  - Item *find_item_searched();
  - Item *find_item_simple();

3. Splitting print() into smaller pieces, adding:

  - void print_when_then_arguments(String *str, enum_query_type query_type,
                                   Item **items, uint count);
  - void print_else_argument(String *str, enum_query_type query_type, Item *item)

4. Moving the maybe_null handling part related to ELSE from fix_length_and_dec()
   to fix_fields(), as in all other Item_func's.

5. Removing the unused String* argument from find_item().

6. Moving find_item() from public to private, as it's not needed outside.

rec_offs_any_flag(): The EXTERNAL and DEFAULT flags are only mutually
exclusive for individual fields. A record can contain both EXTERNAL and
DEFAULT fields, as long as they are separate fields.

sel_restore_position_for_mysql(): Pass the correct index to
rec_is_default_row(). (We should really remove btr_cur_t::index
and use page_cur_t::index isntead.)

This reverts commit d198f961.

The 'default row' record (which stores the default values of
instantly added columns when they were added) must be ignored
during normal operation.

Because btr_pcur_store_position() can theoretically be invoked
on the pseudo-record, we must check every caller of
btr_pcur_restore_position(). Similarly, when positioning the
cursor to the start of an index with btr_pcur_open_at_index_side(),
or when moving to the previous record, we must be careful to
skip the 'default row' record.

rec_is_default_row(): Check whether a leaf page record is the
'default row' pseudo-record.

row_import_set_sys_max_row_id(): Check for a table that is empty but
contains the 'default row' record.

IndexPurge::open(), row_merge_read_clustered_index(),
dict_stats_analyze_index_level(), row_sel_try_search_shortcut(),
row_sel(), sel_restore_position_for_mysql(), row_search_mvcc():
Skip the 'default row' pseudo-record.

fts_get_max_doc_id(): Assert that the 'default row' cannot exist
in the secondary index FTS_DOC_ID_INDEX(FTS_DOC_ID).

page_find_rec_max_not_deleted(): Assert that we are operating
on a leaf page. Also skip the 'default row' pseudo-record, not
only delete-marked records.

row_search_get_max_rec(): Assert that page_find_rec_max_not_deleted()
did not return any delete-marked or 'default row' record.

With instant ADD COLUMN, rec_get_offsets() must reliably know if it
is dealing with a user record (not the page infimum/supremum).

The 'default row' record (which is the only search tuple and the only
leaf-page record that carries the REC_INFO_MIN_REC_FLAG)  must be searched
by using the conventional means. Do not slow down the search shortcut by
taking the MIN_REC_FLAG into account.

rec_offs_validate(): Do not dereference index==NULL.

rec_validate(): Properly handle missing columns.

dict_table_get_nth_v_col(): Assert that no default value is ever set
on a virtual column. Virtual columns are not handled by the
instant ADD COLUMN mechanism.

ha_innobase_inplace_ctx::commit_instant(): Copy the default values
from instant_table to old_table.
Take deep copy of the dict_table_t::v_cols[].

instant_add_instant_try(): Copy the default values to instant_table.

dict_index_get_nth_field_def(): Define the function in the .h file.

dict_col_t::def_val.data: Change the data type to const void*
to reduce the number of type casts.

This could have been done as part of MDEV-11487, which removed the
support for InnoDB internal temporary tables.

The column SYS_INDEXES.MERGE_THRESHOLD was added in MySQL 5.7.
This makes the system table the very first one where
instant ADD COLUMN has been used.

rec_get_nth_cfield(): Remove the heap parameter.
Copying the default value of a field into a heap is only
needed in row_build_low(type=ROW_COPY_DATA).

rec_get_nth_field(): Use the original definition.
There are very few rec_get_nth_cfield() calls compared to
rec_get_nth_field().

Assert that the flags EXTERNAL, DEFAULT, and NULL are mutually exclusive.

Store dict_col_t::def_val inline, using an anonymous struct.

dict_mem_table_fake_nth_col_default(),
dict_col_set_added_column_default(): Remove.

dict_mem_fill_column_struct(): Initialize def_val.