RFR: Add ShenandoahRootProcessor API to report threads while scanning roots

[Roman Kennke]

As discussed in the Traversal GC thread, this breaks out the 
ShenandoahRootProcessor API to report threads while scanning roots. It 
is not implemented here, and only asserts that the ThreadClosure* is 
NULL. All call-sites are updated to pass NULL.

The idea is to make backporting easier/less conflict-prone.

http://cr.openjdk.java.net/~rkennke/root-proc-threads/webrev.00/

Test: hotspot_gc_shenandoah

Ok?