Give a grace period for a dropped execution request

Currently, if an action is queued and not being actively monitored by a client (the client drops the Execute or WaitExecution request) then the action is removed from the queue. However, sometimes a network blip or GKE re-allocation causes the scheduler to drop the connection to the client and therefore cancel the jobs that are still very much required from the queue.

Instead, we should give a grace period of a few seconds for the client to re-connect before removing the action from the queue. Note that this is slightly complicated by the re-connection potentially happening on a different node in a distributed system.