MAJOR: connection : Split struct connection into struct connection and struct conn_stream.

All the references to connections in the data path from streams and
stream_interfaces were changed to use conn_streams. Most functions named
"something_conn" were renamed to "something_cs" for this. Sometimes the
connection still is what matters (eg during a connection establishment)
and were not always renamed. The change is significant and minimal at the
same time, and was quite thoroughly tested now. As of this patch, all
accesses to the connection from upper layers go through the pass-through
mux.
diff --git a/include/types/connection.h b/include/types/connection.h
index 66ec1b6..0d62efc 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -288,9 +288,9 @@
  * data movement. It may abort a connection by returning < 0.
  */
 struct data_cb {
-	void (*recv)(struct connection *conn);  /* data-layer recv callback */
-	void (*send)(struct connection *conn);  /* data-layer send callback */
-	int  (*wake)(struct connection *conn);  /* data-layer callback to report activity */
+	void (*recv)(struct conn_stream *cs);  /* data-layer recv callback */
+	void (*send)(struct conn_stream *cs);  /* data-layer send callback */
+	int  (*wake)(struct conn_stream *cs);  /* data-layer callback to report activity */
 	char name[8];                           /* data layer name, zero-terminated */
 };
 
@@ -347,10 +347,9 @@
 	const struct protocol *ctrl;  /* operations at the socket layer */
 	const struct xprt_ops *xprt;  /* operations at the transport layer */
 	const struct mux_ops  *mux;   /* mux layer opreations. Must be set before xprt->init() */
-	const struct data_cb  *data;  /* data layer callbacks. Must be set before xprt->init() */
 	void *xprt_ctx;               /* general purpose pointer, initialized to NULL */
 	void *mux_ctx;                /* mux-specific context, initialized to NULL */
-	void *owner;                  /* pointer to upper layer's entity (eg: session, stream interface) */
+	void *owner;                  /* pointer to the owner session for incoming connections, or NULL */
 	int xprt_st;                  /* transport layer state, initialized to zero */
 	int tmp_early_data;           /* 1st byte of early data, if any */
 	union conn_handle handle;     /* connection handle at the socket layer */