[dhcpmon] Fix dhcpmon socket filter and tx count issue (#13065)

Why I did it
Fix issue caused by dualtor support PR [dhcpmon] Open different socket for dual tor to enable interface filtering #11201
Improve code
How I did it
On single ToR, packets received count was duplicated due to socket filter set to "inbound"
Tx count not increasing due to filter set to "inbound". Added an outbound socket to count tx packets
Added vlan member interface mapping for Ethernet interface to vlan interface lookup in reference to PR Fix multiple vlan issue sonic-dhcp-relay#27
Exit when socket fails to initialize to allow dhcp_relay docker to restart
How to verify it
Tested on vstestbed single tor and dual tor, sent packets and verify printed out dhcpmon rx and tx counters is correct

Correct number of tx increases
Tx does not increase when ToR is on standby