Create a monotonic clock function, use it where time offsets are computed.

Problem:  If the system time is set backwards while a czmq application is
          running that has timers set on a zloop, those timers will stop
          until the system time reaches its previous value.

Solution: Introduce a monotonic clock that is not affected by system time
          changes.  Use this clock where time offsets are computed.
Merge pull request #566 from mtspencer/master

Create a monotonic clock function, use where time offsets are computed