The InnoDB background DROP TABLE queue is something that we should
really remove, but are unable to until we remove dict_operation_lock
so that DDL and DML operations can be combined in a single transaction.

Because the queue is not persistent, it is not crash-safe. In stable
versions of MariaDB, we can only try harder to drop all enqueued
tables before server shutdown.

row_mysql_drop_t::table_id: Replaces table_name.

row_drop_tables_for_mysql_in_background():
Do not remove the entry from the list as long as the table exists.
In this way, the table should eventually be dropped.