PG::start_peering_interval: always send_notify if !primary

Otherwise, we might get into a situation where the primary
forgets about a stray pg.  This is simpler and does not
increase the number of notifies by much.

Fixes: #7733
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1507 from ceph/wip-7733

PG::start_peering_interval: always send_notify if !primary

Reviewed-by: Sage Weil <sage@inktank.com>