LOCK_open and its derivatives in DML queries".

Re-introduced table cache - a separate cache holding TABLE objects
ready for use by statements protected by its own mutex. In the most
common case a connection opening/closing a table only needs to go
to this cache for getting/releasing TABLE object and do not need
to go to the table definition cache. By having several instances of
such cache (each protected by its own lock) and partitioning
user connections between these instances (based on connection id)
we can greatly improve scalability in some scenarios.

Added new start-up parameter --table_open_cache_instances for
setting number of table cache instances in the system.

Existing --table_open_cache parameter now limits total number of
TABLE object in all table cache instances combined.

Added Table_open_cache_hits, Table_open_cache_misses,
Table_open_cache_overflows - per-connection and global status
variables for tracking and tuning performance of new cache.
(The latter allows to track how often we have to expel tables
from the cache since the table cache instance contains more
than table_open_cache/table_open_cache_instances objects.)