Include node protocol mode in Handshake negotiation for node-to-client protocol

[coot]

Like the network magic is included. This will avoid using Byron or Shelley
when one really needs to run hardfork combinator.

This will be useful in 6-8 weeks when integrating the next hard fork era.

[dcoutts]

For the node-to-client protocol, it'd be nice if we could have the node supply the client with the EpochSlots as part of the handshake. This could eliminate the need to know the EpochSlots up front, which is quite an irritation for clients of the node.

[dcoutts]

Ok, so we have three things here:

1. protocol mode (i.e. Cardano mode, Byron-only mode or Shelley-only mode)
2. protocol mode params (slots per epoch for Byron-only and Cardano mode)
3. network magic

We want to use this differently:

1. The client ought to know this (but it's plausible to imagine examples where it's useful to not know). It should be provided by the client and checked/agreed by the server.
2. The client would really like to not have to know this. The client would like to find out this value as a result of negotiation. The node definitely does know it.
3. The client may know this, but it's also convenient for the client to not have to know and discover as a result of the handshake. The node definitely does know it.

So I think we can handle all these cases if we follow the pattern:

• Client can optionally provide the value
• The node (server) always knows the value
• If the client provides the value, the handshake checks the two agree
• If the client does not provide the value, the handshake informs the client of the correct value

So all these values are not subject to "negotiation" as such in that there are no choices: it's just a case of whether they agree or not.

I think this pattern is compatible with the existing handshake scheme.