osd/: 's/trim_rollback_to/roll_forward_to/g'

osd/: 's/trim_rollback_to/roll_forward_to/g'

trim_rollback_to was a not terrible name before in that all
it ever did is (possibly) trim the stashed version of the
object.  However, now, it's going to encompass, in general,
the roll_forward part of a tpc (which will still be to
delete the stashed object in cases where that is
appropriate).

Signed-off-by: Samuel Just <sjust@redhat.com>