[Enhancement] Add support for unicast/multicast packet identification and add unicast table

[er1cthe0ne]

Context

Current ACA openflow tables doesn't distinguish unicast/multicast packets. It simply sends packets to the multicast table which could flood all the remote tunnel endpoint. While traffic flow still work, but it is not most efficient. The request is to:

1. add the openflow rule to distinguish between unicast/multicast packets.
2. add a unicast table
3. populate the unicast table either by zeta OAM packet, and also create_update_neighbor_port function

Suggestions

1. Try to create the rules by hands and manually run through it with packet to confirm it is working
2. review https://github.com/futurewei-cloud/alcor-control-agent/wiki/Openflow-Tables-Explain

[Zqy11]

I have added unicast/multicast table and modified direct path rule.
The implementation details and test results of unicast/multicast table is in issue-148 chapter 3.1.4/3.2.4 & chapter 5.

[er1cthe0ne]

added in this change in PR: 1fdf845