osd: PeeringState: fix selection order in calc_replicated_acting_stretch
We were previously mis-ordering these to *de*prioritize the existing acting set. That is bad!
We generate OSD candidates from the acting set and strays, and push
them into the candidates list as a tuple of <<!in_acting,pg_info.last_update>,osd_id>.
Then we sort the list. Then we go through the list from front to back and
push_back entries into the appropriate ancestor lists.
And then we pop_back() off the lists to select the acting set.
Which of course turns our nice careful order backwards! So don't do that.