RFR (JDK8-only): Fix freeze on OOM-on-evac regarding the PLL

[Roman Kennke]

This is a complicated one. We may get a freeze under the following
situation:

when the final-mark pause is left, the ShenandoahConcurrentThread sends
a message to the SurrogateLockerThread to release the pending-list-lock 
(see VM_ShenandoahReferenceOperation::doit_epilogue()). The
SurrogateLockerThread is a Java thread that gets kicked off right after
the pause. It attempts to acquire the PLL (a Java lock) and thus
employs a write-barrier on it. When that write-barrier runs out-of-
memory, it ends up in our oom_during_evacuation() loop and is waiting
for the _evacuation_in_progress flag to get cleared. However, since the
ShenandoahConcurrentThread is waiting for the SLT to finish, we never
get to where we clear that flag (we don't even kick off evacuation
yet).

The proposed solution attempts to evacuate the PLL during the pause. If
it succeeds, then the write-barrier will simply pick up the to-space
object. If it fails, we schedule a full-gc, and turn off evacuation
before leaving the pause. In no case can the write-barrier on the PLL
run into OOM, and in all cases will it be correctly unlocked.

Luckily for us, the whole PLL madness has been changed in a very
positive way in JDK9, so this change does not apply there.

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

Ok to push?

Roman