RFR: Refactor ShConcThread dispatch

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/concthread-dispatch-refactor/webrev.01/

A few improvements:
 a) Skip updating the counters if no GC had happened;
 b) Avoid attempting to shrink heap on every tick -- avoids taking ShHeapLock every 10ms;
 c) Code style

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey