Checkpointing Robustness #486
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This makes Arroyo more robust to transient issues with talking to the state backend, namely S3. Previously there were unwraps in the main control loop which could result in the job being suspended. We fix this with two changes. First, the controller will check that a subset of States have an active Receiver. If not, it will be restarted. Secondly, more of the BackendState methods now return Result<()>, which lets us handle these errors.
There are still a number of places where checkpointing errors are fatal, but that is preferable to them reporting as "Running" yet making no progress.