Master should also log server-id collisions

With MDEV-11177 fixed we are now getting duplicate slave id error log messages on the slave side. On the master side either nothing gets logged though, or with log-warnings=3 and a pair of slaves with duplicate IDs the master error log is flooded with messages of the form

2017-08-04 9:42:07 140315054417664 [Note] Start binlog_dump to slave_server(12), pos(master.000001, 520)

several times per second