Use consistent time functions, either from ros or from system

[heuristicus]

Related to #4.

Currently time functions from both ros and system are used, but we should be consistent. The simulation consideration is important, since it will reset ros time to zero if simulated time starts at some point after the roscore starts. Probably don't actually care about this since we are only monitoring processes and not the actual ros system.

This also affects timers since they notice the time jumps when switching between simulated and unsimulated time.Not sure it's possible to avoid getting errors (which don't affect functionality) if using those timers.

roscpp has a wall clock timer, but rospy doesn't. There doesn't seem to be a different library which is easy to use for timers. The event loop does the timer handling at the moment, but ideally we want to move it somewhere where we are not responsible for maintaining it.