This commit implements the new ping/pong messages along with their new
behavior. The new set of ping/pong messages allow clients to generate
fake cover traffic as the ping messages tells the pong message how many
bytes to included and can also be padded itself.
This commit adds Ping and Pong messages to the suite of lnwire
messages. The usage of these messages within the daemon are similar to
the usage of Bitcoin’s ping/pong messages. Pings are to be sent
periodically with a random nonce to check connection activity and also
to gauge latency. Pong’s are to be sent in reply to ping messages,
echo’ing the same nonce used.