RFR: Log message on ref processing and class unload for mark events

[Aleksey Shipilev]

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

Another UX improvement: print marking cycle flavor in the log. Helps to diagnose if failing/slower
marking cycle was somehow special.

[14.017s][info][gc] GC(65) Pause Init Mark 0.266ms
[14.047s][info][gc] GC(65) Concurrent marking (class unload) 900M->961M(1024M) 30.572ms
[14.053s][info][gc] GC(65) Pause Final Mark (class unload) 5.529ms
...
[14.135s][info][gc] GC(66) Pause Init Mark 0.697ms
[14.195s][info][gc] GC(66) Concurrent marking (ref process) 869M->927M(1024M) 60.646ms
[14.196s][info][gc] GC(66) Concurrent precleaning 927M->928M(1024M) 0.431ms
[14.200s][info][gc] GC(66) Pause Final Mark (ref process) 4.355ms
...
[14.378s][info][gc] GC(67) Pause Init Mark 0.633ms
[14.453s][info][gc] GC(67) Concurrent marking 911M->988M(1024M) 75.755ms
[14.456s][info][gc] GC(67) Pause Final Mark 2.735ms

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey