[cdb] Fix gradients going over table header

bin/cdb.css

@@ -88,6 +88,7 @@
 .cdb .cdb-sheet tr.separator td {
   position: sticky;
   top: 19px;
+  z-index: 2;
   border-left: none;
   padding-top: 6px;
   color: #bbb;

bin/cdb.less

@@ -93,6 +93,7 @@
 			td {
 				position: sticky;
 				top: 19px;
+				z-index: 2;
 				border-left: none;
 				padding-top: 6px;
 				color : #bbb;