Removing exchange-v2

[Raphaël Gomès]

Hi all,

The exchange-v2 protocol has been untouched for years now, is incomplete 
and does not seem to have any chance at coming back to life. Its tests 
are unreliable enough to make the CI flaky, which can be quite annoying 
for the developers using it daily (though it's nothing compared to the 
Windows flakiness, but we're working on that).

I thought I'd send this email in case someone was preparing to get back 
on that horse soon, complete the implementation and fix those tests. If 
there are no takers by the end of the month (year!), I'll remove all of 
the code and the associated tests.

Thanks,
Raphaël