Implementing phase 2 of synchronizeReplication.

Phase 2 of synchronizeReplication for split_diff is to
stop all the source 'checker' at a binlog position higher than the
destination master. This has all the plumbing for it.
Almost finishing synchronizeReplication.

Only missing part is the run binlog client until group_id
code, but that requires lower layer changes.
Merge branch 'master' into replication
Merge commit '865c9ff07febad76b56ec75913c75c5dc0ad1866'