WebSocket reliable mode

[jkarneges]

It should be possible to enable a reliable mode for WebSocket connections, similar to what can be done with HTTP connections. The server should be able to supply a prev ID when subscriptions are assigned to connections, and Pushpin should be able to query for missed messages per-channel by last known ID. The feature should work for both WebSocket targets and HTTP targets (WebSocket-over-HTTP).

The big question is what protocol to use for the recovery requests. For WebSocket targets, it could be done using WebSocket messages, however there may be value in having it work over HTTP. For HTTP targets, it could be done using WebSocket messages encapsulated in WebSocket-over-HTTP protocol, but it would be nice if requests could use GET and reuse existing constructs like Grip-Link, Grip-Last, etc.

Note that if the protocol will use WebSocket messages, we'll likely need a way for Pushpin to send control messages to the backend which is not yet possible.