the handlerton will conditionally compile in a reader writer lock for a table share's key_file array in mysql and mariadb versions before 5.5

future versions have metadata locking at the mysql layer, so the handlerton doesn't need the key_file lock.

this change also improves the key_file locking comment and removes some old, non-useful comments that I left behind in the past.



git-svn-id: file:///svn/mysql/tokudb-engine/tokudb-engine@43721 c7de825b-a66e-492c-adef-691d508d4ae1