* SessionLock: reduce the stale time
30 seconds staring at a spinner while we wait for a stale lock to expire is
rather painful. Pretty sure 15 seconds will be fine.
* SessionLock: deal with the clock having been wound back
If a previous session terminated uncleanly, and then the clock is wound back,
we could be waiting a very long time for the previous session's claim to
expire.
We can fix this by simply treating a future claim the same as "now", and
waiting for the normal stale timeout.
* fixup! SessionLock: deal with the clock having been wound back
f3653abe92