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

[Roman Kennke]

Hi Aleksey,

> 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

Looks good, go!

Roman