PG: set info.last_epoch_started on activate even if backfill

If we only find backfill peers, we still want min_last_update_acceptable
to be determined by those backfill peers.  I think this was an oversight
from the acting->actingbackfill switch.

Fixes: #9482
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 231a910d62af765b2772dec064900b46da237b2d)
PG::find_best_info: let history.last_epoch_started provide a lower bound

If we find a info.history.last_epoch_started above any
info.last_epoch_started, we must be missing updates and
min_last_update_acceptable should provisionally be max().

Fixes: #9482
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>