RFR: Update counters on slow-path more rarely

[Aleksey Shipilev]

Running with -UseTLAB, thus stressing shared allocations, is slow because we update the counters
even for the minuscule shared allocations. It makes sense to do this only when new region is claimed
during shared alloc.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/alloc-lazier-counters/webrev.01/

Also removes some superfluous asserts and refactors a bit

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey