CLEANUP: conn_stream: rename cs_endpoint to sedesc (stream endpoint descriptor)

After some discussion we found that the cs_endpoint was precisely the
descriptor for a stream endpoint, hence the naturally coming name,
stream endpoint constructor.

This patch renames only the type everywhere and the new/init/free functions
to remain consistent with it. Future patches will address field names and
argument names in various code areas.
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 8e9bda9..8abd535 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -214,7 +214,7 @@
 	qc_stream_desc_release(qcs->stream);
 
 	BUG_ON(qcs->endp && !se_fl_test(qcs->endp, SE_FL_ORPHAN));
-	cs_endpoint_free(qcs->endp);
+	sedesc_free(qcs->endp);
 
 	eb64_delete(&qcs->by_id);
 	pool_free(pool_head_qcs, qcs);
@@ -1422,7 +1422,7 @@
 	TRACE_LEAVE(QMUX_EV_QCC_END);
 }
 
-static void qc_detach(struct cs_endpoint *endp)
+static void qc_detach(struct sedesc *endp)
 {
 	struct qcs *qcs = endp->se;
 	struct qcc *qcc = qcs->qcc;