osd/PG: use last_epoch_clean from ReadyToMerge point in time for fabricated history

osd/PG: use last_epoch_clean from ReadyToMerge point in time for fabricated history

If we are fabricating the pg history values, we need something that is
reasonably valid, but that won't screw up peering of the PG by indicating
that the PG has peered at some point later than when it really has.
Otherwise we can end up in a situation where everyone thinks there is a
newer pg info out there that doesn't actually exist, and the PG will end
up as incomplete.

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