RFR: System.gc() with ExplicitGCInvokesConcurrent should block

[Aleksey Shipilev]

Hi,

Current ExplicitGCInvokesConcurrent violates the System.gc() contract in this part: "***When control
returns from the method call,*** the Java Virtual Machine has made a best effort to reclaim space
from all discarded objects.". To fix it, we need to wait for the end of cycle before returning from
System.gc() call:
  http://cr.openjdk.java.net/~shade/shenandoah/concthread-eic-block/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey