RFR: Support VerifyBeforeGC and VerifyAfterGC VM options

[Aleksey Shipilev]

On 10/19/2018 07:21 PM, Zhengyu Gu wrote:
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/verify_before_after_gc/webrev.00/index.html

Yeah, well. It is probably the lesser evil to force another safepoint before/after the cycle, rather
than hunt down all the VMOps.

Minor nits:
 *) Move "verify()" declaration to the existing private block in shenandoahControlThread.hpp;
 *) (mode != none) is actually "gc_requested", you can move the conditions in the existing blocks
before and after the switch;
 *) Can we be more symmetrical with other ops, and have ShenandoahHeap::(vm)entry_verify? What would
also allow us to log the verification event;

-Aleksey