trx_undo_update_rec_get_update(): Handle the metadata BLOB correctly.

Correctly calculate the size of mblob when some last fields are missing.

For other than the clustered index, ignore table->instant.

ha_innobase::check_if_supported_inplace_alter(): Do not allow
adding a FTS_DOC_ID column if a hidden FTS_DOC_ID column exists
due to FULLTEXT INDEX. (This always required ALGORITHM=COPY.)

ha_innobase_inplace_ctx::first_alter_pos: The first column whose position
is affected by instant ADD, DROP, or changing the order of columns.

innobase_instant_try(): Fill in a zero default value for the hidden column
FTS_DOC_ID. Before this, it was not possible to instantly add columns if
a hidden FTS_DOC_ID column existed (at the end of the InnoDB table
definition).

instant_metadata(): Create a metadata record for instant ALTER TABLE.

The number of "core fields" must never change unless the whole
index is emptied or rebuilt.

rec_get_converted_size_comp(): Replace status,fields,n_fields with tuple.

rec_init_offsets_comp_ordinary(), rec_get_converted_size_comp_prefix_low(),
rec_convert_dtuple_to_rec_comp(): Add template<bool mblob = false>.
With mblob=true, process a record with a metadata BLOB.

rec_copy_prefix_to_buf(): Assert that no fields beyond the key and
system columns are being copied. Exclude the metadata BLOB field.

dict_col_t::DROPPED: Magic value for dict_col_t::ind

row_metadata_to_tuple(): Convert a metadata record to a data tuple,
based on the new info_bits of the metadata record.

btr_cur_pessimistic_update(): Invoke row_metadata_to_tuple() if needed.

dict_index_t::instant_add_field(): Append fields at the end, if any.
The fields for dropped columns are preserved.

dict_table_t::instant_column(): Renamed from instant_add_column().
Add the parameter col_map so that columns can be reordered.
FIXME: Also support ADD/DROP/reorder/rename of VIRTUAL COLUMN.

ha_innobase_inplace_ctx::prepare_instant(): Preserve any instantly
dropped columns in instant_table and its first index.
FIXME: Support instant ALTER TABLE even if hidden FTS_DOC_ID exists.

innobase_instant_try(): Simplify the logic for detecting dropped
or reordered columns. If any drop or reorder is needed, construct
an update vector with the metadata blob.

dtype_t::metadata_blob_init(): Initialize the metadata BLOB data type.

dict_table_t::find(): Find an old column based on a new column number.

rec_convert_dtuple_to_metadata_comp(): Convert an alter metadata tuple
into a record.

row_rec_to_index_entry_impl(): Add the template parameter mblob
and the optional parameter info_bits for specifying the desired new
info bits. For the metadata tuple, allow conversion between the original
format (ADD COLUMN only) and the generic format (with hidden BLOB).

row_upd_index_replace_metadata(): Apply an update vector to an
alter_metadata tuple.

innobase_add_one_virtual(), innobase_insert_sys_virtual(): Change
the return type to bool, and invoke my_error() when detecting an error.

innodb_insert_sys_columns(): Insert a record into SYS_COLUMNS.
Refactored from innobase_add_one_virtual() and innobase_instant_add_col().

innobase_instant_add_col(): Replace the parameter dfield with type.

innobase_instant_drop_cols(): Drop matching columns from SYS_COLUMNS
and all columns from SYS_VIRTUAL.

innobase_instant_try(): Only invoke innobase_instant_drop_cols() when
there are other changes than adding columns last.

innodb_update_cols(): Renamed from innodb_update_n_cols().

innobase_instant_try(): If any columns were dropped or
reordered, delete any SYS_COLUMNS records for the following
columns, and insert SYS_COLUMNS records for all subsequent
stored columns as well as for all virtual columns.

commit_try_norebuild(): If ctx->is_instant(), let the virtual
columns be added or dropped by innobase_instant_try().

There is no need to omit dropped columns in rec_get_offsets() and
related functions. Dropped columns will have to be dealt with
at a higher level when converting between table rows and
clustered index records.

This is a too specific function, and there is only one caller.

Simplify the code in btr_cur_instant_init_clust_index(),
and make it a part of btr_cur_instant_init_metadata().