DNS standard query response is slow

When use the nslookup command to test DNS, I found the DNS standard query response is slow. So I dump the packet use tcpdump and found the fist DNS standard query always timeout and retry after 5 seconds, and then the response is ok.

I reviewed the corresponding file `vpnservice/natservice/natservice-impl/src/main/java/org/opendaylight/netvirt/natservice/internal/NaptEventHandler.java`,
NaptEventHandler send packetout (set inPort and action as OFPP_TABLE so that it starts from table 0) after install the INBOUND_NAPT_TABLE/OUTBOUND_NAPT_TABLE.

But it can't insure the installed INBOUND_NAPT_TABLE/OUTBOUND_NAPT_TABLE flows existed before the packetout.