[krbdev.mit.edu #3935] CCAPI implement locking

[Jeffrey Altman via RT]

Implement server side locks on context and ccache objects.  Use a timer
to track how long the lock was outstanding.  If the lock is held by a
client for more than XXX seconds and the client holding the lock has not
been heard from during that time period, blow away the lock and permit
the operation.  (Perhaps invalidate all of the client's contexts or just
the iterators.)