MaxScale hangs with warning about "Worker 1 attempted to send a message to worker 1"

An existing bug in older 2.5 versions manifested as a lost event. In 2.5.24 this manifests as a hang which will, under intensive workloads, end up cause the SystemD watchdog to kill MaxScale.

The reason for this is the inappropriate use of internal messages for tasks which other mechanisms should be used. Previously this misuse manifested as an error about the failure to write a message to a worker which also caused hanging client connections.