Make the binlog dump threads not need to take LOCK_log while sending
binlog events to slave. Instead, a new LOCK_binlog_end_pos is used
just to coordinate tracking the current end-of-log.

This is a pre-requisite for MDEV-162, "Enhanced semisync
replication". It should also help reduce the contention on LOCK_log on
a busy master.

Also does some much-needed refactoring/cleanup of the related code in
the binlog dump thread.