Implement SSHStore on top of nix-daemon served over ssh

[shlevy]

Fixes #986

[shlevy]

@edolstra ping, will merge this at the end of the week absent any issues

[edolstra]

Please don't, I'll want to review this first.

[shlevy]

OK, I'll count that as an issue 😄

[shlevy]

@edolstra Can you give me a time frame for reviewing this? It's the most controversial change in the perl->c++ work and if I'm going to have to redo build-remote without it I'd rather know sooner than later...

[shlevy]

(erm, and nix-copy-closure)

[shlevy]

@edolstra Is there anything I can do to help get this reviewed? If I'm going to have to redo build-remote and nix-copy-closure without this it will be much easier now than in a few weeks (though still a pain now of course).

[edolstra]

To reduce the churn in this PR and make it easier to review, could you not rename RemoteStore to DaemonStore? We can always rename things sensibly later...

Also, wrt build-remote, the use of a separate protocol allows the use of a forced SSH command to root where the client can only do a few operations (copying paths and starting builds). How would that work here?

[shlevy]

OK, will do with the rename.

For build-remote, we could add a limited mode to the daemon? What do you think?

[edolstra]

Yeah that would be good. It would probably suffice to add a flag that causes the daemon to use "nobody" as the connecting user name rather than using the uid of the calling user.

[shlevy]

Replaced by #1053