osd: simplify master log recreation; fix up Log::copy_after

osd: simplify master log recreation; fix up Log::copy_after

Pull log from a given point from peer with the largest last_update.  Do
not worry about divergence on the peer; that is handled by the new
primary.  Simplifies PG::Query struct.

Fix copy_after to set an accurate .bottom, and to behave if the split
point given is divergent (i.e. doesn't actually appear in the log).