RFR: Log concurrent mark that updates references

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/mark-message-ur/webrev.01/

Small follow-up, we can actually print if we are running CM-with-UR or not.

Testing: hotspot_fast_gc_shenandoah, eyeballing logs

Thanks,
-Aleksey