In commit 8f8ba758 (MDEV-27234)
the data dictionary recovery was changed to use READ COMMITTED
so that table-rebuild operations (OPTIMIZE TABLE, TRUNCATE TABLE,
some forms of ALTER TABLE) would be recovered correctly.

However, for operations that avoid a table rebuild thanks to
being able to instantly ADD, DROP or reorder columns, recovery
must use the READ UNCOMMITTED isolation level so that changes to
the hidden metadata record can be rolled back.

We will detect instant operations by detecting uncommitted changes
to SYS_COLUMNS in case there is no uncommitted change of SYS_TABLES.ID
for the table. In any table-rebuilding DDL operation, the SYS_TABLES.ID
(and likely also the table name) will be updated.

As part of rolling back the instant ALTER TABLE operation, after the
operation on the hidden metadata record has been rolled back, a rollback
of an INSERT into SYS_COLUMNS in row_undo_ins_remove_clust_rec() will
invoke trx_t::evict_table() to discard the READ UNCOMMITTED definition
of the table. After that, subsequent recovery steps will load and use
the correct table definition.

Reviewed by: Thirunarayanan Balathandayuthapani
Tested by: Matthias Leich