WL#3035: Optimize OPTIMIZE of an open HANDLER table

Things like 'optimize table' / 'alter table' shall be able to close all handler
tables that are not in use. This can be done in the following way:
- When accessing 'thd->handler_tables_hash' have a lock on LOCK_open.
- Add a marker in the table object that the table is locked (should already
exists, but better to verify that this is reset properly on unlock tables).
- In remove_table_from_cache(), call mysql_ha_flush_table(...
MYSQL_HA_REOPEN_ON_USAGE) for all handler tables that are not locked.