osd: simplify osdmap tracking of osd up/down epochs; fix pg build_prior logic

osd: simplify osdmap tracking of osd up/down epochs; fix pg build_prior logic

Use a single struct to track all of our osd up/down info.  Include
down_at, the epoch we last marked the osd down.

Fix PG::build_prior to require that the osd was clean through the _entire_
interval in question.

In monitor, adjust new clean interval foward to down_at-1 if the up_from
matches the interval we mounted.  That is, if the OSD shut down cleanly,
it obviously remained clean at least until we marked it down in the map.