Using a boolean flag for 'there is a RESET MASTER in progress' doesn't
work very well for multiple concurrent RESET MASTER statements.
Changed to a counter.
During server shutdown, we need to wait for binlog checkpointing to
finish in the binlog background thread before closing the binlog.
This was not done, so we could get assert and failure to finish the
final binlog checkpoint if shutdown happened in the middle.
Make the commit checkpoint inside InnoDB be asynchroneous.
Implement a background thread in binlog to do the writing and flushing of
binlog checkpoint events to disk.
Introduce a new storage engine API method commit_checkpoint_request().
This is used to replace the fsync() at the end of every storage engine
commit with a single fsync() when a binlog is rotated.
Binlog rotation is now done during group commit instead of being
delayed until unlog(), removing some server stall and avoiding an
expensive lock/unlock of LOCK_log inside unlog().
