Remove some unnecessary computations and conditions.

FIXME: Instant add/drop/reorder operations on stored columns
cannot yet be combined with operations on virtual columns.

innobase_instant_drop_col(): Remove the parameter for the table name.
The InnoDB data dictionary could be out of sync with the .frm file,
because renaming columns between upper and lower case may not be
reflected by the InnoDB dictionary.

innobase_op_instant_try(): Remember the number of the last stored
column, and support dropping multiple columns at a time.
FIXME: adjust SYS_COLUMNS.POS for the pre-existing (not added)
virtual columns.

dict_col_t::add_to_core(): Convert an instantly added column
to a "core column". The old name remove_instant() could
be mistaken to refer to "instant DROP COLUMN".

dict_col_t::is_added(): The new name for is_instant().

- Optimised the remove_instant if there is no dropped column present.

Clarify some comments about accessing an externally stored column
on which a spatial index has been defined. Add a TODO comment that
we should actually write the minimum bounding rectangle (MBR) to
the undo log record, so that we can avoid fetching BLOBs and recomputing
MBR.

row_build_spatial_index_key(): Split from row_build_index_entry_low().

can handle for dropped columns too.
- Restricted the drop column with FTS_DOC_ID and
hidden fts index removal for instant operation.

Do not exclude instantly dropped columns from the size of the record.
This has to be handled at the upper level, by row_build() and the
to-be-written replacement of row_build_index_entry(), called
row_build_clust_index_entry().

Hopefully only a small number of functions will have to directly
access those clustered index record fields that can be reordered
or instantly dropped.

Until row_build() is taking care of reordering the fields in the
clustered index record, we cannot really support anything else
than instant DROP COLUMN of the last columns of a table.

rec_info_bits_valid(): Remove. The only case when this would fail
is when the record is the generic ALTER TABLE metadata record.

rec_is_alter_metadata(): Check if a record is the metadata record
for instant ALTER TABLE (other than ADD COLUMN). NOTE: This function
must not be invoked on node pointer records, because the delete-mark
flag in those records may be set (it is garbage), and then a debug
assertion could fail because index->is_instant() does not necessarily
hold.

rec_offs_validate(): FIXME: Temporarily disabled an assertion
that does not hold if rec_is_alter_metadata(). Because we do not know
here if a ROW_FORMAT=REDUNDANT record is for the leaf page, we cannot
invoke rec_is_alter_metadata() here.

- Changed the condition for is_instant() for dict_index_t

- Changed the metadata row format by adding number of null bytes in the record.
- Changed the checking condition for instant alter in check_if_supported_inplace_alter()
- Fixed the instant_alter test case

Allow combination of non-instant, non-rebuilding operations with
changes of table options that do not require a rebuild.

For example, DROP INDEX or ADD INDEX can be performed with
ALGORITHM=NOCOPY together with changing such table options.
Changing the table options alone would be allowed with ALGORITHM=INSTANT.

INNOBASE_ALTER_NOCREATE: A new set of flags, for operations that
are refused for ALGORITHM=INSTANT and do not involve creating
index trees.

Move ALTER_RENAME_INDEX to the proper place (INNOBASE_ALTER_INSTANT).

innobase_need_rebuild(): Do not require a rebuild if
INNOBASE_ALTER_NOREBUILD operations are combined with ALTER_OPTIONS.

ha_innobase::prepare_inplace_alter_table(),
ha_innobase::inplace_alter_table(): Use the fast path if
ALTER_OPTIONS is combined with INNOBASE_ALTER_NOCREATE.
In this case, the actual changes would be deferred to
ha_innobase::commit_inplace_alter_table().

On instant DROP COLUMN, there is no point in updating indexes
that depended on the dropped column. This avoids a memory access
violation in innodb.instant_alter.

Also, clarify terminology: alter_metadata instead of new_default_row.

This follows up to commit 755187c8.

TRX_UNDO_INSERT_METADATA: Renamed from TRX_UNDO_INSERT_DEFAULT

trx_undo_metadata: Renamed from trx_undo_default_rec

For instant ALTER TABLE, we store a hidden metadata record at the
start of the clustered index, to indicate how the format of the
records differs from the latest table definition.

The term 'default row' is too specific, because it applies to
instant ADD COLUMN only, and we will be supporting more classes
of instant ALTER TABLE later on. For instant ADD COLUMN, we
store the initial default values in the metadata record.