MDEV-27783 InnoDB: Failing assertion: table->get_ref_count() == 0 upon ALTER TABLE ... MODIFY COLUMN

- There is a race condition occurs between purge thread and DDL.
So purge thread can increment n_ref_count even after DDL does
purge_sys_t::stop_FTS().

- dict_table_open_on_id for purge thread should check
purge_sys.must_wait_FTS() before acquring the table.

- purge_sys.stop_FTS() does acquire dict_sys.latch for setting
the purge system flag and check table ref count on auxilary tables.