zketcd: support deleting directories left empty by ephemeral nodes

Ephemeral node deletion doesn't update the CVersion, so the directory
will seem to have children when it doesn't after losing ephemeral nodes.

Note: this fails cross-checking and etcd likely needs an API extension
to get full zk compatibility.

Fixes #88

zketcd.go

@@ -213,13 +213,15 @@ func (z *zkEtcd) mkDeleteTxnOp(op *DeleteRequest) opBundle {
 			}
 		}
 
-		crev := s.Rev(mkPathCVer(p))
+		// Force CVer into read-set to catch any conflicting update
+		// which would invalidate emptiness check.
+		s.Rev(mkPathCVer(p))
+		// Check if directory has any children.
 		gresp, gerr := z.c.Get(z.c.Ctx(), getListPfx(p),
 			// TODO: monotonic revisions from serializable
 			// etcd.WithSerializable(),
 			etcd.WithPrefix(),
 			etcd.WithCountOnly(),
-			etcd.WithRev(crev),
 			etcd.WithLimit(1))
 		if gerr != nil {
 			return gerr