Eliminate potential race conditions while rebooting

[cbgbt]

Issue number:
Closes #630

Description of changes:
This PR best left in commit-order, as it also performs several maintenance chores.

    agent: eliminate potential races while rebooting
    
    The agent had a few conditions under which it can enter the
    MonitoringUpdate state before it successfully uncordons the node:
    
    * Brupop updates its state into Rebooted before the reboot terminates
      the process
    * After rebooting, an error ocurrs after updating the state but before
      performing the uncordon.
    
    We avoid this conditions by:
    * Exiting the agent process in cases where the desired state is Rebooted
      but we are not yet running the new version.
    * Always uncordoning the node before marking that we have successfully
      transitioned into the Rebooted state.
    * Defensively uncordoning the node once again when we enter the
      Monitoring state.

Testing done:
I built a custom Bottlerocket variant which always waits for a minute before performing a reboot, then used this to test these changes. In doing this, I was able to reliably trigger the race condition, and then also confirm that this new code does not misbehave when reboots occur slowly.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.