Add naive bidirectional close detection AND add debug log to stdout

- TcpBidirectionalFlowKey, a new struct type is used to associate
directional flows with bidirectional connections.

- FlowTrack now uses TcpBidirectionalFlowKey as the key to an additional
map of NFQueueTraceRoute`s...

- The goroutine which reads the ICMP packets off the wire...
I have modified it to also read TCP packets and perform a naive connection
close detection.