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>
Merge pull request #2519 from athanatos/wip-9482

PG::find_best_info: let history.last_epoch_started provide a lower bound

Reviewed-by: Sage Weil <sage@redhat.com>