[Unison-hackers] version negotiation

[Tõivo Leedjärv]

Sorry for spamming, this will be the last one for today.

I believe I have a working solution to automatically detect that both
server and client are >= 2.51.5 and then proceed with protocol upgrade
and negotiation. I will clean up the code and push into the PR.

It is somewhat of an ugly solution but since it only matters with
already released versions then we can be sure about how they behave
(still, must get plenty of field testing this time). The solution
relies on aspects of the protocol that do not depend on OCaml version.

This covers the last bit of the puzzle. All scenarios will now
automatically detect >= 2.51.5 compatibility and switch to new
protocol negotiation. This means that the server default can be <
2.51.5.