MEDIUM: streams: Always create a conn_stream in connect_server().

MEDIUM: streams: Always create a conn_stream in connect_server().

In connect_server(), when creating a new connection for which we don't yet
know the mux (because it'll be decided by the ALPN), instead of associating
the connection to the stream_interface, always create a conn_stream. This way,
we have less special-casing needed. Store the conn_stream in conn->ctx,
so that we can reach the upper layers if needed.