Helix Incorrectly Reports Pending Messages (potentially)

We use the following MBean to track Helix Pending messages in Pinot Servers:

```
CLMParticipantReport:MonitorType=ParticipantMessageMonitor,ParticipantName=some_instance_id:
```

We have often seen that this MBean reports that there are Helix pending messages, but there's no thread processing any Helix messages. There are also no messages in `INSTANCES//MESSAGES` ZNode.

In this particular instance, I see the following things happened around the time we saw this issue:

* There was a big GC pause
* Zk client lost connection
* There's a Helix log message: `Tasks that never commenced execution after 200`. This is followed by a list of 238 FutureTask, and the MBean is set to exactly 238 Pending Messages. There are also log lines which indicate that the `HelixTaskExecutor` had shutdown.
* There were failures in deleting Helix messages in `INSTANCES//MESSAGES`. There were 9 such messages.

Some other signals can be seen in the attached Grafana screenshot.