Network support: adding networkinng paramters to messages.

[intigonzalez]

Since we are dealing with network protocols, we need a way to specify "low-level" details about how messages are sent and received. For instance, we need a mechanism to specify transport protocol and port number. Below, the initial proposal we already discussed:

messages
    // local messages
    gossip_time: field, another_field
    hello_time 
    new_rumor_time
    // remote messages
    network(protocol:tcp, port:156, keep-alive: true) {
        hello: field, another_field
    }
    network(protocol:tcp, port:156, out:reuse) {
        rumor
    }

    network(protocol:tcp, port:156) {
        hello: field, another_field
        rumor
    }