In today's unattended upgrade, we have seen how the prewarming pool size dropped to (near) zero. This was based on lost Nomad allocations. The allocations got rescheduled, but not added again to Poseidon.
The reason for this is a miscommunication between the Event Handling and the Nomad Manager. `removedByPoseidon` was true even if the runner was not removed by the manager, but an idle runner.
Destroying the runner when Nomad informs us about its allocation being stopped, fixes the error of executions running into their timeout even if the allocation was stopped long ago.
When allocations fail and restart they are added again to the idle runners. The bug fixed with this commit is that the inactivity timer was not stopped at the restart. This led to the idle runner being removed when the timer expired.
