MEDIUM: mux-quic: subscribe on xprt if remaining data after send

The streams data are transferred from the qcs.buf to the qcs.xprt_buf
during qc_send. If the xprt_buf is not empty and not all data can be
transferred, subscribe the connection on the xprt for sending.

The mux will be woken up by the xprt when the xprt_buf will be cleared.
This happens on ACK reception.

src/mux_quic.c

@@ -197,6 +197,11 @@ static int qc_send(struct qcc *qcc)
 
 			fprintf(stderr, "%s ret=%d\n", __func__, ret);
 			qcs->tx.offset += ret;
+
+			if (b_data(buf)) {
+				qcc->conn->xprt->subscribe(qcc->conn, qcc->conn->xprt_ctx,
+				                           SUB_RETRY_SEND, &qcc->wait_event);
+			}
 		}
 		node = eb64_next(node);
 	}