Expose server_connection functions to user

[ada2k]

This allows for the user to use httpcats while writing their own server loop, which is essential for a number of usecases including parsing of the tcp PROXY protocol or custom logging code/logic that must run before a request is parsed. There is also a small change to the signature of handlers to avoid an assert false when H2 is impossible.

[dinosaure]

Not sure to understand the goal of this PR but I just added the ability to manipulate the underlying flow into the server handler (needed when we use the CONNECT method and try to implement a proxy with httpcats).

[dinosaure]

I see the goal when I figure out only the mli. I made an issue to express what this PR wants to solve.