regularly clean up closed SESSIONS entries

[martinpitt]

The running pods and sessions map in redis can go out of sync for any number of reasons: session pods crashing and the "websocket EOF" message not making it through, or API pods crashing at the wrong time, or new sessions in state wait_target which were never connected to. In the app service

• track the time of session creation in SESSIONS
• do a regular sweep over:
  • all running session pods, and delete the ones which are not in SESSIONS
  • all SESSIONS entries, and delete the wait_target ones which are older than 1 hour, and the closed ones, and the running ones which are older than, say, one day?

[martinpitt]

Retitling. Our deployer service account in k8s cannot remove pods, and there is no other standard account that can. In PR #61 I let the session containers die by themselves. So this reduces to cleaning up the SESSIONS map.