Add a start/stop lifecycle for ServerCommandClientImpl

[kwvanderlinde]

Identify the Bug or Feature request

Fixes #5052

Description of the Change

ServerCommandClientimpl can now be stopped when the MapToolClient is closed. This shuts down the TimedEventQueue thread, in turn allowing the ServerCommandClientImpl, MapToolClient and Campaign to be garbage collected.

Some minor related edits:

• Removed an unncessary synchronized block from TimedEventQueue.
• Fixed AssetProducerThread.shutdown() to use an AtomicBoolean instead of a plain boolean.

Possible Drawbacks

None

Documentation Notes

N/A

Release Notes

• Fixed a bug where unloaded campaigns would remain in memory.`

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