server: Compare MAC tags in constant time.

server: Compare MAC tags in constant time.

This fixes a timing attack.  If an adversary can watch the timestamp on
the server's log, then it might be possible to determine how much of a
forged packet's MAC is invalid, and thereby figure out one byte at a
time.

This requires the new constant-time comparison function in Catacomb, so
update the dependencies.

This is release 1.0.0pre13.