RFR: Common pause marker to capture everything before/after pause

[Roman Kennke]

Am 11.09.2017 um 20:49 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/pause-marks/webrev.01/
> 
> Our current pauses handling is inconsistent: some of them do not set up IsGCActiveMark, some of them
> miss record_gc_(start|stop). It makes sense to have a common "marker" for all pauses, and deal with
> everything that is needed to be done there.
> 
> Testing: hotspot_gc_shenandoah
> 
> The failure in TestShenandoahStrDedup prompted me to check the safepoint, hence the TODO.
> 
> Thanks,
> -Aleksey
> 

This looks good and useful. Go!