Use futures efficiently

[FractalBoy]

Rather than peppering code with calls to Future->retain(), this returns a future back to the server for it to await.

In some cases, returning a future would be too difficult, so we call ->get or ->await to get the result.

Additionally, now that we can return a future to the server, it should be able to process requests that have already been read into the read buffer faster (and present in $buffref), since calling ->get or ->await will only return control to the main server read function when there is more data to be read (not when the data has already been read but not parsed).