RFR: Clean up and regroup Shenandoah flags

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/cleanup-flags/webrev.01/

This is actually a few changesets together:

 *) Remove obsolete unused flags:
      ShenandoahHappyCyclesThreshold, ShenandoahFailHeapExpansionAfter

 *) Remove ShenandoahWriteBarrierRB flag:
      No need for newly shaped WB; only assemblers have the use, which is not necessary

 *) Rename ParallelSafepointCleanupThreads to ShenandoahParallelSafepointThreads:
 *) Rename VerifyObjectEquals to ShenandoahVerifyObjectEquals:
      Shenandoah-specific flags should be prefixed appropriately

 *) Clean up and regroup Shenandoah flags:
      Cleans up formatting;
      Rehashes some comments;
      Makes all options experimental;
      Makes all options read-only (we don't test that writes actually work correctly)
      Groups options logically;

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey