Fully implement SRS callback API

[JanKoppe]

We should implement a full SRS callback API so that we get as much integration and feedback from SRS as possible.

Currently a very minimal, naive approach for this exists in the RTMP app.

Documentation of the HTTP hooks that are implemented in SRS can be found here: https://github.com/ossrs/srs/wiki/v3_EN_HTTPCallback

Note that the on_play and on_stop features will not necessarily be useful for us in their original purpose. We do not intend to let viewers connect to the stream via RTMP against SRS, but instead front it all behind HLS with a caching layer. Therefore these on_play and on_stop events will be triggered by internal clients, e.g. restream, transcode, or other specialized functions.

If possible, always try to find a layer of abstraction where we could offer a slightly modified version that works with e.g. nginx-rtmp or other possible streaming servers.