MINOR: connection: Add a function to detach a conn-stream from the connection

cs_detach() function is added to detach a conn-stream from the underlying
connection. This part will evovle to handle applets too. Concretely,
cs_destroy() is split to detach the conn-stream from its endpoint, via
cs_detach(), and then, the conn-stream is released, via cs_free().
diff --git a/include/haproxy/connection.h b/include/haproxy/connection.h
index e0af4c3..169cd58 100644
--- a/include/haproxy/connection.h
+++ b/include/haproxy/connection.h
@@ -336,6 +336,7 @@
 	cs->obj_type = OBJ_TYPE_CS;
 	cs->flags = CS_FL_NONE;
 	cs->conn = conn;
+	cs->ctx = conn;
 }
 
 /* returns 0 if the connection is valid and is a frontend connection, otherwise
@@ -390,8 +391,12 @@
 	conn->subs = NULL;
 }
 
-/* Release a conn_stream */
-static inline void cs_destroy(struct conn_stream *cs)
+/* Detach the conn_stream from the connection, if any. If a mux owns the
+ * connection ->detach() callback is called. Otherwise, it means the conn-stream
+ * owns the connection. In this case the connection is closed and released. The
+ * conn-stream is not released.
+ */
+static inline void cs_detach(struct conn_stream *cs)
 {
 	if (cs_conn(cs)) {
 		if (cs->conn->mux)
@@ -409,6 +414,13 @@
 			conn_free(conn);
 		}
 	}
+	cs_init(cs, NULL);
+}
+
+/* Release a conn_stream */
+static inline void cs_destroy(struct conn_stream *cs)
+{
+	cs_detach(cs);
 	cs_free(cs);
 }