Access repository over SFTP transport

[sourcefrog]

Following on from #13, access the repository over SFTP, probably using https://docs.rs/ssh2/.

This is mostly implemented in https://github.com/sourcefrog/conserve/tree/sftp.

The code for this works in a branch, but not on the released version of SSH2, because of https://github.com/alexcrichton/ssh2-rs/issues/251.

Releasing this in Conserve was blocked on https://github.com/alexcrichton/ssh2-rs/issues/278, a new release of SSH2 that includes the fixes for readdir, but that is now fixed.

Still to do:

• [x] Use the released version of ssh2.
• [ ] Merge up to master.
• [ ] Make the dependency optional?
• [ ] Document that SSH support exists
• [ ] Set a default remote username the same as the local username: at present the default is "" which is rejected with an unclear message
• [ ] At least, document that authentication is only from an ssh-agent
• [ ] Add some option to permit authentication other than by an agent: a key file, password, etc?

Alternatively, SSH could be supported based on https://docs.rs/openssh-sftp-client/latest/openssh_sftp_client/ and https://docs.rs/openssh/0.10.1/openssh/. That would

• Only work on Unix.
• Respect all your local configuration of SSH, since it literally runs external ssh.
• Authenticate in whatever way is configured in to SSH, without needing every method to be specifically supported in Conserve.

[sourcefrog]

There is now https://docs.rs/openssh-sftp-client/latest/openssh_sftp_client/ which might be more promising:

• Uses external ssh to authenticate so will inherit configuration
• May be more actively maintained?