RFR: Refactor allocation seqnum handling: split mutator/GC allocs, merge with other metadata handling

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/alloc-seqnums/webrev.01/

This refactors allocation seqnum machinery, so that we have more data to feed the heuristics with.
Notably, this should help Roman's work on Traversal GC that wants to ignore some of the newly
allocated regions.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey