Measure the time spent outside processing the Source and the Event
 - between the end of the previous Event and the beginning of the Source
 - between the Source and the current Event
Note: this will include the time spent in per-Run and per-Lumisection modules
fix accounting of pre-Source time for the first event
Merge pull request #225 from fwyzard/FastTimerService-pre_post_source-62X

FastTimerService pre/post-Source for 6.2.X