change behavior of drainGracePeriod to be smoother for brief outages

[sawall]

Currently drainGracePeriod() causes a hard down if prosody is marked unhealthy due to a health check timeout. We should change this behavior to go into drain if a timeout is causing the issue with prosody.

drain currently kicks in if:

we've ever been healthy and
jicofo is unhealthy but otherwise all else is good and
the current time is less than window ending at first failure time + grace period

[sawall]

@aaronkvanmeerten @bgrozev

proposed criteria for hard down (down immediately) vs soft down (drain for grace period then hard down):

• hard down if jicofo times out or returns a 500 (usually meaning no JVBs available)
• soft down if jicofo returns 503 because it times out in its own prosody health checks
• hard down for a 5xx from prosody
• soft down for prosody timeouts