RFR: Disable complete bitmap verification in init mark

[Aleksey Shipilev]

Basically, our verifier spends >50% of time dealing with dead allocations now.
It seems excessive to verify them before Init Mark, because it will rebuild the
bitmaps anyway. After conc mark is done, it will blow most immediate garbage,
and subsequent verification phases are good. Same reasoning applies to Full GC.

Patch:
  http://cr.openjdk.java.net/~shade/shenandoah/verifier-perf-new-allocs/webrev.01/

Thanks,
-Aleksey