Update dhcpmon rx/tx packet filtering and fix server rx count (#13898)

Why I did it
Dhcpmon had incorrect RX count for server side packets. It does not raise any false alarms, but could miss catching server side packet count mismatch between snapshot and current counter.

Add debug mode which prints counter to syslog

How I did it
Due to dualtor inbound filter requirement, there are currently two filters, each for listening to rx / tx packets.
Originally, we opened up an rx/tx socket for each interface specified, which causes duplicate socket. Now we initialize the sockets only once. Both sockets are not binded to an interface, and we use vlan to interface mapping to filter packets. For inbound uplinks, we use a portchannel to interface mapping.

Previous dhcpmon counter before dual tor change:
[ Agg-Vlan1000- Current rx/tx] Discover: 1/ 4, Offer: 1/ 1, Request: 3/ 12, ACK: 1/ 1
[ eth0- Current rx/tx] Discover: 0/ 0, Offer: 0/ 0, Request: 0/ 0, ACK: 0/ 0
[ eth0- Current rx/tx] Discover: 0/ 0, Offer: 0/ 0, Request: 0/ 0, ACK: 0/ 0
[ PortChannel104- Current rx/tx] Discover: 0/ 1, Offer: 0/ 0, Request: 0/ 3, ACK: 0/ 0
[ PortChannel103- Current rx/tx] Discover: 0/ 1, Offer: 0/ 0, Request: 0/ 3, ACK: 0/ 0
[ PortChannel102- Current rx/tx] Discover: 0/ 2, Offer: 1/ 0, Request: 0/ 6, ACK: 1/ 0
[ PortChannel101- Current rx/tx] Discover: 0/ 0, Offer: 0/ 0, Request: 0/ 0, ACK: 0/ 0
[ Vlan1000- Current rx/tx] Discover: 1/ 0, Offer: 0/ 1, Request: 3/ 0, ACK: 0/ 1
[ Agg-Vlan1000- Current rx/tx] Discover: 1/ 4, Offer: 1/ 1, Request: 3/ 12, ACK: 1/ 1

Dhcpmon counter after this PR:
[ PortChannel104- Current rx/tx] Discover: 0/ 1, Offer: 0/ 0, Request: 0/ 3, ACK: 0/ 0
[ PortChannel103- Current rx/tx] Discover: 0/ 1, Offer: 0/ 0, Request: 0/ 3, ACK: 0/ 0
[ PortChannel102- Current rx/tx] Discover: 0/ 2, Offer: 1/ 0, Request: 0/ 6, ACK: 1/ 0
[ PortChannel101- Current rx/tx] Discover: 0/ 0, Offer: 0/ 0, Request: 0/ 0, ACK: 0/ 0
[ Vlan1000- Current rx/tx] Discover: 1/ 0, Offer: 0/ 1, Request: 3/ 0, ACK: 0/ 1
[ Agg-Vlan1000- Current rx/tx] Discover: 1/ 4, Offer: 1/ 1, Request: 3/ 12, ACK: 1/ 1

How to verify it
Ran dhcp relay test to send all four packets in singles and batches on both single ToR and dual ToR. Counter was as expected.