Parser should return how many bytes are needed.

[reitermarkus]

Currently, when the buffer length is < 6, None is returned. Instead, the parser should return the minimum number of bytes needed to progress, similar to nom's Err::Incomplete variant.

[lkolbly]

Thanks! Yes, the current parser I think can be greatly improved. I have a somewhat lower-level parser which I prototyped (see protocol::BufferHeadContents in the lkolbly/parse-buffer branch). Once we have a better handle on dealing with multiple protocol versions, I think this'll be the next thing I look at.