add MonotonicTimer

[flixr]

based on MonotonicTime (which uses the CLOCK_MONOTONIC).
This timer is not influenced by time jumps of the system time,
so ideal for things like periodic checks of timeout/heartbeat, etc...

Depends on ros/roscpp_core#55

[flixr]

So while in theory this should work well, it seems that there is a bug in the boost condition timed_wait:
Even if you pass it a duration it will simply add it to the current system time and wait until then.
It seems to be fixed in boost 1.61 if you specify BOOST_THREAD_HAS_CONDATTR_SET_CLOCK_MONOTONIC, see https://svn.boost.org/trac/boost/ticket/6377

So that means that currently the MonotonicTimer can nicely deal with forward jumps in time, but not with backwards jumps.

Possible solutions:

• also only wait for 1ms just like in the sim time case and re-check the sleep_end predicate
  • drawback is that you up a lot more often than needed
  • to do this only for the MonotonicTimer we either need to add a member function isMonotonicTime or use a typeid check?
• replace the boost condition variable with C++11 equivalent where you can specify it to use a monotonic clock

[flixr]

Closing in favor of #1014