subsystem. Fix a number of caveates that the previous
implementation suffered from, including unprotected
access to shared data and lax resource accounting
(share->ref_count) that could lead to deadlocks.

The new implementation still suffers from a number
of potential deadlocks in some edge cases, and this is 
still not enabled by default. Especially since performance
testing has shown that it gives only marginable (not even 
exceeding measuring accuracy) improvements.

@todo: 
- Remove calls to close_cached_tables() with REFRESH_FAST,
and have_lock, because they break the MDL cache. 
- rework FLUSH TABLES <list> to not use close_cached_tables()
- make sure that whenever we set TABLE_SHARE::version to
0 we free MDL cache references to it.