RFR: Avoid indirection to next-mark-context

[Roman Kennke]

On top of the ShenandoahMarkingContext patch, this change avoids the
extra indirection that we always need to take in hot marking path to get
to the next-mark-bitmap and next-TAMS-structures. It does so by picking
the next-marking-context at the beginning of the phase, and then keep it
around in the closure.

Testing: tier3_gc_shenandoah ok

http://cr.openjdk.java.net/~rkennke/direct-marking/webrev.00/

Ok?

Roman