MEDIUM: connection: make mux->detach() release the connection

For H2, only the mux's timeout or other conditions might cause a
release of the mux and the connection, no stream should be allowed
to kill such a shared connection. So a stream will only detach using
cs_destroy() which will call mux->detach() then free the cs.

For now it's only handled by mux_pt. The goal is that the data layer
never has to care about the connection, which will have to be released
depending on the mux's mood.
diff --git a/include/proto/connection.h b/include/proto/connection.h
index cae5dc3..4c952e0 100644
--- a/include/proto/connection.h
+++ b/include/proto/connection.h
@@ -694,12 +694,7 @@
 /* Release a conn_stream, and kill the connection if it was the last one */
 static inline void cs_destroy(struct conn_stream *cs)
 {
-	struct connection *conn = cs->conn;
-
-	LIST_DEL(&conn->list);
-	conn_stop_tracking(conn);
-	conn_full_close(conn);
-	conn_free(conn);
+	cs->conn->mux->detach(cs);
 	cs_free(cs);
 }
 
diff --git a/src/mux_pt.c b/src/mux_pt.c
index e9a7d2f..8a8aec0 100644
--- a/src/mux_pt.c
+++ b/src/mux_pt.c
@@ -113,11 +113,16 @@
 }
 
 /*
- * Detach the stream from the connection
- * (Used for outgoing connections)
+ * Detach the stream from the connection and possibly release the connection.
  */
 static void mux_pt_detach(struct conn_stream *cs)
 {
+	struct connection *conn = cs->conn;
+
+	LIST_DEL(&conn->list);
+	conn_stop_tracking(conn);
+	conn_full_close(conn);
+	conn_free(conn);
 }
 
 static void mux_pt_shutr(struct conn_stream *cs, enum cs_shr_mode mode)