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.

src/wormhole/server/rendezvous.py

@@ -10,8 +10,8 @@ HOUR = 60*MINUTE
 DAY = 24*HOUR
 MB = 1000*1000
 
-CHANNEL_EXPIRATION_TIME = 3*DAY
-EXPIRATION_CHECK_PERIOD = 2*HOUR
+CHANNEL_EXPIRATION_TIME = 2*HOUR
+EXPIRATION_CHECK_PERIOD = 1*HOUR
 
 def generate_mailbox_id():
     return base64.b32encode(os.urandom(8)).lower().strip(b"=").decode("ascii")