MEDIUM: peers: make use of stream_accept_session()

MEDIUM: peers: make use of stream_accept_session()

Instead of going through some obscure initialization sequences, we now
rely on the stream code to initialize our stream. Some parts are still
a bit tricky as we cannot call the frontend's accept code which is only
made for appctx in input. So part of the initialization past the stream
code is what ought to be in the frontend code instead. Still, even
without this, these are 71 lines that were removed.