mon/PGMap: cache min_last_epoch_clean

We call this a lot.  Only recalculate when we decode or when we apply
an incremental.

Adjust unit test accordingly.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/PGMap: only recalculate min_last_epoch_clean if incremental touches old min

If the Incremental updates a value that used to equal the old min, we may
have raised it and need to recalculate it at the end.  Otherwise, we can
avoid recalculating at all!

Signed-off-by: Sage Weil <sage@inktank.com>