upparat: limit exponential backoff max time

[livioso]

This mitigates #9.

Till now, the backoff would just keep increasing. Not entirely sure what's an appropriate max_value to choose since we have vastly different use cases (could fail due to auth issues, could fail because the connection is flaky ...). Nonetheless, I would argue it makes sense to set it.

Any concerns with 64s?

closes #9.

[aeby]

64s probably is on the lower end, but with not that many devices I guess it is ok.

[philippbachmann08]

LGTM
64s might be low for certain scenarios like Soest where bandwith seems to be limited. But for now it's not the plan to update too many devices at once