osd,mon: keep last_epoch_started along with last_epoch_clean premerge

osd,mon: keep last_epoch_started along with last_epoch_clean premerge

We need to populate both the last_epoch_clean and last_epoch_started
accurately when fabricating a pg merge target.  In the case where the
premerge PG peers, recovers (e.g., backfill), then becomes ready to
merge, the last_epoch_clean may be > the last_epoch_started, and using
the later last_epoch_clean will prevent the PG from subsequently
peering (it'll go to incomplete state instead).

Fix by keeping both values, separately.

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