Add more lldpa supports

Reviewer: @rlane @harshsin

1st draft
Summary:
For TX_REQ, it sends out LLDP PDU for a specified interval
For RX_REQ, it expects to receive incoming LLPD PDU for a specified interval. Send Timeout Msg to the controller LLDP PDU is unexpected (both data and timing)
Implement both oftests and unit tests