RFR: Refactor/consolidate/cleanup

[Roman Kennke]

We have several pieces of information duplicated, mostly between
ShenandoahHeapRegion and fast-access variants in ShenandoahHeap:

- in-collection-set
- top-at-mark-start (prev and next)

This change consolidates the code to use only the fast-access versions.
There's no reason to have both, and struggle to keep them in sync.

Swapping mark bitmap now also swaps the corresponding top-at-mark-start 
pointers. Before this was done in different places and was quite a
mess.

prev- and next marking bitmap and top-at-mark-start pointers are
renamed to 'complete' and 'next', this tells a bit more about what they
are: the 'next' structures are only ever used during marking, it's the
bitmap in progress. The 'complete' structures are completed markings.
The 'next' gets swapped with 'complete' right after marking.

I removed marked_next_object_iterate(). We should only ever use
complete marking bitmaps for iteration. This was an exception during
mark-compact, because we did not swap the bitmaps after marking.

Also removed a bunch of unused and obsolete code.

Tested by jtreg and SPECjvm2008

http://cr.openjdk.java.net/~rkennke/bigrefactor/webrev.00/

Ok?

Roman