RFR: Disable merged cleanup for full GC

[Roman Kennke]

In ShenandoahMarkCompact, we temporarily install a special BarrierSet
that disables read-barriers. However, this conflicts with deflating
monitors during full-gc because this requires functional read-barriers
(e.g. if a monitor points to a from-space object, this needs to be
resolved). The 'solution' for now is to disable merged monitor deflation
for full GC:

http://cr.openjdk.java.net/~rkennke/disable-merged-cleanup-fullgc/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/disable-merged-cleanup-fullgc/webrev.00/>

This solves the problem that Aleksey reported with WeakReferenceTest in
jcstress.


Ok?