Alter handshake to allow fixed/variable fields

[cwgoes]

At least remote identifier, version.

Then optimistic handshakes can fix fields, non-optimistic can leave variable, if identifiers are unimportant they can be auto-selected.

[ethanfrey]

Please specify if this is for channel or connection handshakes, or both?

I see the link to connOpenTry issue, so I assume for connections. Funny, as I was just about to write up a similar issue for channels. But I'll make a new issue

[cwgoes]

No reason not to support it in both handshakes, I think.

[cwgoes]

Ideally we could do this without additional fields... maybe a sentinel value of empty-string?

Seems like empty-string shouldn't really be a valid identifier or version in the usual case.