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: I049149d2c47051805aa3164f947b0572af597ac7
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",
     };