… unreachable because it was not promoted

…re it seems

+ moved where we are calling mark_ro_segments to the right places -
  for BGC this needs to be in the 2nd non concurrent phase)
  for blocking GCs I just moved it to a more appropriate place
+ for full blocking GCs it's not enough to do mark_ro_segments when gen start seg is not ephemeral.
when we are doing a gen2 GC, even if we have acquired a new seg for that heap, we still need
to mark all in range ro segs because our logic in IsPromoted expects when there's in range ro
segs we need the mark bit to tell us these ro objects are marked.
+ got rid of some unnecessary checks

Note that I do make the assumption that ro segs are always threaded at the beginning of gen2. We
actually always thread these into heap 0's gen2 seg list but I can see a chance of that changing,
if we want a better balancing of the mark_ro_segments work.

…isabled via env.var but we'll get rid of that soon)