RFR: Explicit concurrent GC should do traversal when Traversal GC is enabled

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/traversal-explicit-gc/webrev.01/

If user supplies -XX:+ExplicitGCInvokesConcurrent when "traversal" is enabled, and the application
calls System.gc(), we would enter normal concurrent cycle. Which is obviously not what we want, at
all (it should go down in flames, actually).

Testing: hotspot_gc_shenandoah, added test cases

Thanks,
-Aleksey