Problem:
=======
The "Start binlog_dump" message hasn't been updated to include the slave's
requested GTID position:

20:05:05 139836760311552 [Note] Start binlog_dump to slave_server(2), pos(, 4)

For diagnostic purposes, it would be helpful if the GTID position were
included.

Fix:
===
Imporve "Start binlog_dump" print message to include "using_gtid" and
"GTID position" requested by slave.

Ex:
[Note] Start binlog_dump to slave_server(2), pos(, 4), using_gtid(1),
  gtid('1-1-201,2-2-100')

[Note] Start binlog_dump to slave_server(3), pos('mariadb-bin.004142',
  507988273), using_gtid(0), gtid('')