half protocol implementations

[Geal]

Right now, there's a high coupling between front and back sockets in most proxy implementations, especially due to the BufferQueue implementation and the HTTP implementation.

For HTTP/2 it would be useful to separate protocol halves, ie:

• server for HTTP 1 front
• client for HTTP 1 back
• server for HTTP/2 front
• client for HTTP/2 back
• pipe (should be the same for both front and back)

We could still keep some specific implementations, to keep good performance for the HTTP 1 only proxy, or to reintroduce TCP splicing.

This work would pave the way for easy integration of new protocols, like GRPC or MQTT.

The most pressing issue: separating front and back HTTP 1 implementations. We will then be able to reuse parts of it for backend connections in the HTTP/2 proxy.

[Geal]

done. The configuration now uses 16393 as default buffer size, since HTTP/2 requires a minimal frame size of 16384, with 9 more bytes for the header