Made a some new buffers thread specific and changeable.
Resize of key_buffer.
AUTO_COMMIT -> AUTOCOMMIT
Fixed mutex bug in DROP DATABASE
Fixed bug when using auto_increment as second part of a key where first part could include NULL.
Split handler->extra() to extra() and extra_opt() to be able to support thread specific buffers.
Don't write message to error log when slave reconnects becasue of timeout.
Fixed possible update problem when using DELETE/UPDATE on small tables
(In some cases we used index even if table scanning would be better)
A lot of minior code cleanups
