Remove update_event as a single event loop for the system

[bmc-msft]

Builds on #159

TODO:

• Move major components into their own timer loops (task/job in one, etc)
• Update Task timer loop to pull from the task heartbeat queue (requires Heartbeat updates)
• Update Node timer loop to pull from the node heartbeat queue (requires Heartbeat updates)
• Merge pool resize & scaleset timer loops into a single loop
• Ensure tasks stop after their duration
• Ensure jobs stop after their duration
• Ensure repro stop after their duration

[bmc-msft]

The remaining todo items will be done in a separate PR once #164 is finished.