Re-connection mechanic

[CxRes]

We need a re-connection mechanic when subscription is dropped to ensure that no notifications were lost in the interim.

A possible candidate is re-using "Last-Event-ID" mechanism from SSE/Eventsource. The state of the notification can be sent back using the Last-Event-ID header upon reconnection, so the resource sender can take appropriate action: Send missing notifications, inform you to re-fetch the resource etc.