MAJOR: conn_stream/stream-int: move the appctx to the conn-stream

MAJOR: conn_stream/stream-int: move the appctx to the conn-stream

Thanks to previous changes, it is now possible to set an appctx as endpoint
for a conn-stream. This means the appctx is no longer linked to the
stream-interface but to the conn-stream. Thus, a pointer to the conn-stream
is explicitly stored in the stream-interface. The endpoint (connection or
appctx) can be retrieved via the conn-stream.