Introduce status variables for table cache monitoring and tuning

1. Since MariaDB's definition and use of table_open_cache_instances is different from MySQL's, it naturally calls for a status variable which would indicate how many instances are in use at the moment. MySQL does not have it, but it's not needed there, because there table_open_cache_instances shows exactly that.

2. MySQL 5.6/5.7 has status variables Table_open_cache_hits, Table_open_cache_misses, Table_open_cache_overflows. Even though MariaDB's table cache tuning is more automated than MySQL's, the variables can still be useful in MariaDB too.