GTID update may block the REST-API

The mariadbmon monitor sends updates to all the routing threads about the GTID positions of the servers it monitors. These updates end up being sent via the same thread that also handles the REST-API traffic which end up blocking the REST-API if one of the worker threads is busy doing something. As was seen in MXS-4685, this could lead to a slowdown of the REST-API even for operations that normally do not involve collecting data from the individual worker threads.

This problem was already fixed for the 23.08 release where the slowdown was detected but since it uses mechanisms that exist in 2.5, the fix should be backported there.