RFR: Implement flag to generate write-barriers without membars

[Roman Kennke]

I extracted this from Traversal GC because it may be useful for other 
situations too. It introduces a flag ShenandoahWBWithMembar which 
enables to avoid generation of the load-load-membar in the 
write-barrier. This membar is not needed when evacuation is always 
turned off at safepoints (e.g. partial).

http://cr.openjdk.java.net/~rkennke/wbwithmembar/webrev.00/

Test: hotspot_gc_shenandoah passed

Roman