server/keyexch.c: Lower the validity time for a challenge.

server/keyexch.c: Lower the validity time for a challenge.

If the server is busy dealing with LAN-speed traffic, it can easily
exhaust the 32 MB data limit within the 2 minutes allowed.  The result
of this is that another switch or switch-ok gets sent and ignored, and
no new keys are negotiated.  The only thing to do is lower the validity
time.  One key-exchange every 20 seconds isn't going to break the bank.