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.
Change the name of fields of messages which are belong to the discovery
subsystem in a such way so they were the same with the names that are
defined in the specification.
In this commit announcement signature message has been added which is
needed when peers want to announce their channel to the rest of the
network. This message acts as half proof carrier, nodes exchanges
their half proofs with each other and after that they are able to
construct the full proof.