rewrite read_body to carry conn forward

[mwhitworth]

fixes #175

with some caveats:

• this removes the behaviour where {:stream, X} was returned. It was not clear whether this ever worked with any other adapter than HTTPoision - Investigate restoring streaming request body through to upstream #176 tracks the work here. But we are now storing in memory request bodies that may have previously been streamed.
• the read_body interface (the return value) changes, but this shouldn't be considered part of the public interface for the plug