RFR: Control loop should wait before starting another GC cycle

[Aleksey Shipilev]

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

Following up on some test timeouts on our large test servers. It seems that running with
"aggressive" on lots of CPUs and small heap occupancy stalls single-threaded workloads enough for
them to timeout. So, it makes sense to make the interval between successive GC cycles. We currently
have one, but it only applies to idle phases. The change above does it on all paths.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey