HealthMonitor displays ExecutorManager queue statistics

The HealthMonitor now displays various qeueue statistics of the ExecutorManager.
This is done to help figure out which queues are taking up a lot of space (so
which are potentially overloaded).
Merge pull request #1048 from pveentjer/improvement/2.x/healthmonitor-executormanagerqueues

HealthMonitor displays ExecutorManager queue statistics