RFR [11], 2018-07-30, Bulk backports to sh/jdk11

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20180730/webrev.01/

This backports latest work to sh/jdk11:

 [backport] Skipping null strings when popping dedup string candidates
 [backport] Trace and report total allocation latency and sizes
 [backport] -XX:-UseTLAB should disable GCLABs too
 [backport] Exponential thread-local GCLAB sizing
 [backport] Elastic TLABs may avoid peeking into the free set racily
 [backport] Refactoring ShenandoahStrDedupStress test to reduce test time
 [backport] Optimize traversal mark: constify, avoid polling traversal set and is_marked_next
 [backport] Traversal should resize TLABs
 [backport] Refactor to group marking bitmap and TAMS structure in one class ShenandoahMarkingContext
 [backport] Refactor alive-closures to deal better with new marking contexts
 [backport] Avoid indirection to next-mark-context
 [backport] TLAB sizing policy should converge faster with Shenandoah
 [backport] Enable worker scaling for ref-proc
 [backport] Move periodic GC decision making to GC heuristics base class
 [backport] Fix single-threaded reference processing
 [backport] Improve C2 barrier filtering: heap-stable check should test for HAS_FORWARDED only
 [backport] Fix task queue assertion with traversal GC
 [backport] Reset TAMS for uncommitted empty regions to avoid leftover old values
 [backport] Handle missing ShenandoahWriteBarrierRB case
 [backport] Move Shenandoah stress tests to tier3

Testing: tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey