expire channels faster: 2 hours, not 3 days
expire channels faster: 2 hours, not 3 days

With increased usage, I'm seeing a buildup of stale channels. Since the
channels aren't properly ephemeral yet (where they get closed as soon as
the last subscriber disconnects), clients which terminate without
calling close() tend to leave the channel lying around. We don't have
"persistent wormholes" yet, so channels should be much more ephemeral
than they currently are.

warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole