Decide of the reliability and ordering guarantees of notifications (and implement them)

[tomaka]

For context, see the discussion here: paritytech/substrate#6692 (comment)

It is unclear at the moment whether the notifications sent using NetworkService::write_notification or NetworkService::notification_sender provide some guarantee that the notifications will reach their destination and in the same order as they have been sent.

99.9% of the time the notifications will indeed reach their destination and in the correct order. The problematic situations are corner cases, most notably the situation where we have multiple connections open, and one of them closes.

cc @romanb