RFR: Refine ShenandoahEvacLockGranularity default and bounds check

[Roman Kennke]

I propose to raise the default of ShenandoahEvacLockGranularity to 6,
this appears to be a sweet spot in almost all of the workloads that
I've thrown at it.

This patch also checks bounds of the option to avoid locks that span
more than one region. Includes testcase.

http://cr.openjdk.java.net/~rkennke/evac-lock-args/webrev.00/

Testing: hotspot_gc_shenandoah

I intend to push this to shenandoah/jdk (if HG server lets me - it
seems to be overdosed at the moment), then give it a few runs in CI,
and if all is well, propose it for jdk/jdk.

Roman