RFR: Full GC should not always update references

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/fullgc-no-ur/webrev.01/

This resolves one of the TODOs, and makes Full GC faster because of that. We are sure this is
correct, because asserts in phase1 would detect unexpectedly forwarded objects. New Verifier code
would also assert has_forwarded bits are correctly (re)set.

Testing: tier3_gc_shenandoah, benchmarks with "passive"

-Aleksey