RFR (S) 8247845: Shenandoah: refactor TLAB/GCLAB retirement code

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8247845

Fix;
  http://cr.openjdk.java.net/~shade/8247845/webrev.01/

Current TLAB/GCLAB retirement code is all over the place. Sometimes we retire GCLABs twice.
Sometimes we resize TLABs twice. This hopefully makes the things more clear by lifting things out of
CollectedHeap::ensure_parsability and specializing it for Shenandoah use cases.

Testing: hotspot_gc_shenandoah {fastdebug,release}; tier{1,2} with Shenandoah; benchmarks (running)

-- 
Thanks,
-Aleksey