On rollback of changes to SYS_COLUMNS, we can no longer assume that
the change was the instant addition of a column at the end of the
column list. The function dict_table_t::rollback_instant(unsigned n)
was inherently incompatible with instantly dropping or reordering
columns.

When a change to SYS_COLUMNS is rolled back, we must simply evict
the affected table definition, at the end of the rollback. We cannot
free the table object immediately, because the current transaction
that is being rolled back may be holding locks on the table.

dict_table_remove_from_cache_low(): Replaced
by dict_table_remove_from_cache().

dict_table_remove_from_cache(): Add a third parameter keep=false,
so that the table can be freed by the caller.

dict_table_t::rollback_instant(): Add the parameter old_n_fields,
and simplify some logic.

trx_lock_t::evicted_tables: List of tables on which trx_t::evict_table()
was invoked.

trx_t::evict_table(): Evict a table definition during rollback.

trx_commit_in_memory(): Empty the trx->lock.evicted_tables list
after the locks were released, by freeing the table objects.

row_undo_ins_remove_clust_rec: Invoke trx_t::evict_table() on the
affected table if a change to SYS_COLUMNS is being rolled back.

row_undo_mod_clust_low(): FIXME: do the same