RFR: Backport PLAB stuff to shenandoah/jdk10

[Roman Kennke]

http://cr.openjdk.java.net/~rkennke/backport-plabs/webrev.01/

We can't use the ThreadLocalData stuff, so it keeps the _gclab field and
accessors in Thread. Other than that it is mostly straightforward
backport. It still reduces shared code changes significantly (in
CollectedHeap, Thread and ThreadLocalAllocBuffer).

Testing: hotspot_gc_shenandoah (pending, but looking good so far)

Ok?

Roman