PG: set info.last_epoch_started on activate even if backfill

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' and 'origin/wip-9481' into wip-sam-testing