RFR: FinalEvac pause to turn off evacuation

[Roman Kennke]

We've had plenty of small and big bugs surrounding our turning off
evacuation concurrently at the end of evacuation. And we jump through
some hoops in C2 to insert a load-load-membar to ensure ordering of
loading the flag and loading the fwd pointer is correct. We can forgo
all of this by turning evac off at a safepoint always. This was
previously difficult because the flag would also participate in
OOM-during-evac-handling, but this is fixed now too. This patch
implements that, and removes all the crap to support concurrently
turning off evac:

http://cr.openjdk.java.net/~rkennke/final-evac/webrev.00/

Passes: hotspot_gc_shenandoah

Ok?