RFR(S) 8224932: Shenandoah: Rename ShenandoahHeapLock, make it general purpose lock

[Zhengyu Gu]

Please review this refactor ShenandoahHeapLock to general purpose spin 
lock (I kept ShenandoahSpinLock, cause ShenandoahLock was taken in 
another changeset. Any other suggestions for the name?)

Bug: https://bugs.openjdk.java.net/browse/JDK-8224932
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224932/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu