RFR (XS) 8241520: Shenandoah: simplify region sequence numbers handling

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241520

Webrev:
  https://cr.openjdk.java.net/~shade/8241520/webrev.01/

It ditches the seqnums we don't really need, and make the remaining one Traversal-specific. (We can
remove it when/if Traversal goes away).

Testing: hotspot_gc_shenandoah {fastdebug,release}; torture tests for allocation path improve a bit

-- 
Thanks,
-Aleksey