fixed a race condition with the expirer thread

basically writes to any member of the command structure which is accessed
by the expirer thread need to be protected using the command->lock.

Other kind of accesses don't need to be synchronized since the rest of the logic
makes use of the main kepaxos instance lock ... but that can't be used by the expirer
thread because it's running inside an iterator and would dead lock
the thread sanitizer blesses us