RFR (S): 8188919: Generalize GC thread suspend/resume at safepoints

[Erik Österlund]

Hi,

When we synchronize/unsynchronize safepoints, we make sure to perform 
corresponding suspend/resume calls to GC-specific concurrent workers 
that may interfere with the safepoint. This is currently done by dumping 
GC-specific callbacks right in SafepointSynchronize::begin() and 
SafepointSynchronize::end(). The purpose with this cleanup is to hide 
this behind a general interface on CollectedHeap instead.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8188919/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8188919

Thanks,
/Erik