RFR: Make Traversal share code with conc-mark

[Roman Kennke]

As noted in the original review thread for the traversal mode, a bunch
of stuff is an exact copy from conc-mark, and remained so since then.
I am about to fix something in liveness accounting (to spread liveness
for humongous objects over its part regions), and I don't want to fix
it in two places, so here comes a change that removes the traversal
copies and makes it use conc-mark code instead.

The only difference was in an assert that I weakened very recently. I
moved that over into the corresponding sh-conc-mark code with an
additional is_conc_traversal_in_progress() guard.

http://cr.openjdk.java.net/~rkennke/traversal-share-concmark/webrev.00/

Passes hotspot_gc_shenandoah

ok?