Update task policy for contrail-dns.

When XMPP channel with agent clsoes, the records exported from there
are removed in the cleaner task context. Update the task policy to
ensure the config and bind tasks do not run in parallel with it.

Change-Id: Ic92f147060c39e452742aa67ace7e3a6f3ddc5a3
closes-bug: 1533811

src/dns/cmn/dns.cc

@@ -81,6 +81,9 @@ void Dns::SetTaskSchedulingPolicy() {
                          exclude_io);
 
     const char *garbage_exclude_list[] = {
+        "dns::Config",
+        "dns::BindStatus",
+        "db::DBTable",
         "bgp::Config",
         "xmpp::StateMachine",
     };