Remove the extra class hierarchy with classes TC_LOG_queued, TC_LOG_unordered,
and TC_LOG_group_commit, folding the code into the TC_LOG_MMAP and
TC_LOG_BINLOG classes. In particular TC_LOG_BINLOG is greatly simplified by
this, unifying the code path for transactional and non-transactional
commit.

Remove unnecessary locking of LOCK_log in MYSQL_BIN_LOG::write() (backport
of same fix from mysql-5.5).