print debug information if debug flag is on

note: packet name is only set if NODE_DEBUG variable is set

would be good to benchmark if there is any visible performance impact if debug flag is _not_ on

TODO: add output format to readme
Incoming packet:
 `==> CommandName#stateName(seqNo, packetType, packetLen)`
Outgoing packet:
 `<== CommandName#stateName(seqNo, packetName, packetLen)`

(packet type in the incoming packet is because we only can reliably know from packet header if it's Error or EOF packet, other types depend on state and cannot be detected using packet content. For outgoing packet we know name )