Add general use RTMP server

[varsill]

This PR:

• adds a separate RTMP server capable of serving multiple clients

TODO:

• [x] Rewritte Membrane.RTMP.Source in such a manner that it uses the RTMP.Server
• [x] Add support for SSL socket
• [ ] Update docs and examples

To run the server, use the following command:

Membrane.RTMP.Server.run(port: 1935)

and start some RTMP client, e.g. with the use FFmpeg:

ffmpeg -r 30 -f lavfi -i testsrc -vf scale=1280:960 -vcodec h264 -profile:v baseline -pix_fmt yuv420p -f flv rtmp://localhost:1935/live/test