negotiate P2P protocol version for tor remotes

negotiate P2P protocol version for tor remotes

This negotiation is not supported by versions of git-annex older
than 6.20180312. Well, maybe really 6.20180227 or so, but using that in
the changelog simplifies things since it was the version for the other
changes as well.

See commit c81768d425725f868ddf23333d49eed0f3fda011 for the back story.

As well as allowing for future protocol improvements, this will result
in negoatiating protocol version 1, which is an improvement over default
version 0.

In fact, it looks like no supported version of git-annex will use
protocol version 0, since version 1 was introduced in 6.20180227.
Still, removing the code for version 0 seems unncessary.
See commit 31e1adc005e8181c5190a9f39a3f148e10f4d364.

Sponsored-by: Brett Eisenberg on Patreon.