Implement flags to allow routing messages over specific peers

[chris--]

Extend the message format to include a via and reply-via flag to make sure a message passes a specific peer on its way back to the sender.

{
    to: '123abc',
    from: 'cba321',
    type: 'some-proto',
    payload: (...),
    reply-via: 'def123'
}

If peer 123abc receives a message with reply-via set, it answers with a message where the via-field is set to the value of reply-via like so:

{
    to: '123abc',
    from: 'cba321',
    type: 'some-proto',
    payload: (...),
    via: 'def123'
}

[chris--]

As an alternative, we can include a reference to the original peer in the payload. The decision where to route the message to must then be taken by the registered protocol.

{
    to: '123abc',
    from: 'def123',
    type: 'some-proto',
    payload: {
        origin: 'cba321',
        (...)
    }
}

An answer to this message could look like this:

{
    to: 'def123',
    from: '123abc',
    type: 'some-proto',
    payload: {
        receiver: 'cba321',
        (...)
    }
}

When 123abc receives the message, the application protocol decides to relay the payload to the origin cba321.

[makkes]

Since the 'via' field is only useful in the routing protocol itself we better implement it as part of the payload of the routing protocol. Otherwise we would have to hold state in the router for every incoming message that has a 'via' field set (to set it correctly in the answer).