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.  The bug was introduced when
we started activating missing object sources in the patch below.

Fixes: #9482
Backport: firefly
Introduced: 84e2f39c557c79e9ca7c3c3f0eb0bfa4860bf899
Signed-off-by: Samuel Just <sam.just@inktank.com>
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>