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 remote-tracking branches 'wonzhq/req-pending', 'origin/wip-9326', 'somnathr/wip-optracker-optimization', 'origin/wip-8315', 'upstream/wip-7767-b', 'origin/wip-9453', 'origin/wip-9481', 'origin/wip-9482' and 'origin/wip-9497' into wip-sam-testing