RFR: Move Verifier "start" message under (gc,start)

[Aleksey Shipilev]

This unclutters the GC log a bit:
  http://cr.openjdk.java.net/~shade/shenandoah/verifier-start-msg/webrev.01/

Sample log:

[9.484s][info][gc] GC(4) Verify Before Mark, Level 4 (1297209 reachable, 0 marked)
[9.485s][info][gc] GC(4) Pause Init Mark 135.297ms
[9.609s][info][gc] GC(4) Concurrent marking 539M->640M(876M) 124.407ms
[9.609s][info][gc] GC(4) Concurrent precleaning 640M->640M(876M) 0.118ms
[9.826s][info][gc] GC(4) Verify After Mark, Level 4 (1536381 reachable, 1280120 marked)
[9.830s][info][gc] GC(4) Pause Final Mark 640M->625M(876M) 220.602ms
[9.912s][info][gc] GC(4) Concurrent evacuation  625M->747M(876M) 82.028ms
[9.912s][info][gc] GC(4) Concurrent reset bitmaps 747M->747M(876M) 0.171ms


Thanks,
-Aleksey