Support a sane WebSocket protocol

[enkore]

Ideally we want to support the REST API, the FCM push service and websockets in one server with as little hassle as possible (previous hassle).

Estimate := 3 in the assumption that e.g. Tornado's websockets (or what we'll be using, but Tornado is nice and we have great experience with it on the blockserver) doesn't explode as hard.

Sane := not a one way trip, and doesn't require one WebSocket connection per drop the client wants. Instead, something like this conceptually (> := client-to-server, < := server-to-client)

> subscribe drop_id [drop_ids...]
...
< drop drop_id message
> post drop_id message
< drop drop_id message
< drop other_drop_id message
...
> unsubscribe  drop_id [drop_ids...]

(note: "post drop_id message" can be implement with almost no code by just forwarding it to the REST API - it has the advantage that it doesn't require the client to make another connection to POST)

When TLS (wss://) is used this also makes it harder to monitor everything, since drops are multiplexed

(note 2: uWSGI does heartbeating, there is no need at all to replicate heartbeats in the app protocol)

[enkore]

Simplifications:

• One WebSocket connection per drop
• Server->client required, client->server optional
• Very simple protocol; one drop per data frame. Could be as simple as binary frames with everything up to the first \n\n being the X-Qabel-Since stuff, and everything after that a drop message. But we'll have to look at how this turns out, depending on Impl it very well may be that here the impl dictates the spec for now.