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

[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