MINOR: h3: add h3c pointer into h3s instance

As a mirror to qcc/qcs types, add a h3c pointer into h3s struct. This
should help to clean up H3 code and avoid to use qcs.qcc.ctx to retrieve
the h3c instance.
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 04ed1ba..84237c3 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -139,7 +139,7 @@
 
 	qcs->id = qcs->by_id.key = id;
 	if (qcc->app_ops->attach) {
-		if (qcc->app_ops->attach(qcs))
+		if (qcc->app_ops->attach(qcs, qcc->ctx))
 			goto err;
 	}
 
@@ -434,7 +434,7 @@
 {
 	TRACE_ENTER(QMUX_EV_QCS_RECV, qcc->conn, qcs);
 
-	if (qcc->app_ops->decode_qcs(qcs, qcs->flags & QC_SF_FIN_RECV, qcc->ctx)) {
+	if (qcc->app_ops->decode_qcs(qcs, qcs->flags & QC_SF_FIN_RECV)) {
 		TRACE_DEVEL("leaving on decoding error", QMUX_EV_QCS_RECV, qcc->conn, qcs);
 		return 1;
 	}