GTID values not updated if only slaves are present

When MaxScale is configured to monitor the last two slaves (S2 and S3) of a four node chained cluster:

M <- S1 <- S2 <- S3

The GTID values in the maxctrl list servers are not shown when the slave servers have read-only on. If read-only is not enabled, S2 is labeled as the master and the GTIDs are updated.

This is caused by the requirement to know the master's domain ID before the GTID update is done (line 1814 in mariadbmon.cc). By unconditionally updating the GTID positions of the servers, the GTIDs are updated correctly even when only slaves are being monitored.

[server3]

type=server

address=127.0.0.1

port=3002

protocol=MariaDBBackend

[server4]

type=server

address=127.0.0.1

port=3003

protocol=MariaDBBackend

[MariaDB-Monitor]

type=monitor

module=mariadbmon

servers=server3,server4

user=maxuser

password=maxpwd

monitor_interval=2000

[CLI]

type=service

router=cli

[CLI-Listener]

type=listener

service=CLI

protocol=maxscaled

socket=default