Starting BinlogServer in replica vttablets.
Interrupting services when binlog is disabled.
Adding tests for binlog server state.

And fixing a small bug, don't want to serve if state is disabled.
Small cleanup before bigger cleanup.
Changing _vt.blp_checkpoint to not have uid.

Now the primary key is keyspace_start, keyspace_end.
Also simplifying vt_binlog_player to get ready to move
it to mysqlctl.
Merge commit '9f6d43c1fd5729c42b2b46aba2fb4e1b57b86848'
Merge commit 'ccd09431daf502bad07d0d255be38f5f95e005c7'
Merge commit '06f41612b4ae16e28da132a875023dde28159eea'
Merge commit '920d4e0688c94da75de358ee603c21ac7db48afd'
Merge commit '322f9e3922d1ed938abbd372ab1fd08a31a786aa'