Reconnect sftp connection

[ibash]

Draft PR for #353

TODO:

• Cleanup
• Thread safety
• More testing

The retry mechanism indicates that 10 retries should be done. But it seems like more than 10 reconnections are done, I'd like to find out why.

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[MichaelEischer]

• Thread safety

You should probably start with a mutex around the reconnect() method. But you'll probably need a way to prevent multiple reconnections, if multiple uploads fail for the same reason.

• The retry mechanism indicates that 10 retries should be done. But it seems like more than 10 reconnections are done, I'd like to find out why.

The retries are per file. Restic by default uploads up to 5 pack files in parallel. This will probably need some upper limit for the number of reconnections.

In general, reconnecting should probably be opt-in as not everyone has setup password-less ssh.