Dispatch read one byte of the code, so a code with a non-zero high byte
was handled as its low byte. Message::get_message_code now reads the
whole little-endian uint32 and handlers register by u32. PeerInit and
PierceFirewall, the only frames with a one-byte code, get their own
accessor for the listener's first frame.
Peer message code 50 had no handler, so the dispatcher dropped it and a
download refused by the peer stayed Queued until the caller's timeout.
The new handler reads the filename and reason and routes the denial into
the same failure path as UploadFailed. A reason of "Queued" is status,
not a failure: some clients send it for a download that is still held in
their queue, and failing it would kill healthy downloads after a
reconnect, so that one is only logged.
Fixes#29
Michel de Graaf