Event simulator II

The previous event simulator simulated time by making all tasks that
would have been launched as threads run in the same thread. That only
worked to a point but didn't properly simulate more complex behaviors
such as clustering.

This new one handles things properly by still running tasks in
eventlet's greenthreads but forcing them to only run one at a time.

Change-Id: Ife834e8d55193da8416ba700e55e7b0c2496f532