RFR (XS): Interleave "process references" and "unload classes" to amortize the pause

[Aleksey Shipilev]

Hi,

Obvious idea: make sure reference processing and class unloading do not happen
on the *same* N-th cycle to amortize the costs. Otherwise we make the "outlier"
pause even more outlier-ish.

Webrev:
 http://cr.openjdk.java.net/~shade/shenandoah/heuristics-interleave-class-refs/webrev.01/

Testing: hotspot_gc_shenandoah

-Aleksey