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* core
* `commands.execute()`: avoid reporting on exception twice; send notification message on details of retry attempt (614)
* `settings.IORetrySettings`: add ServerDisconnectedError and FSTimeoutError to retryable exceptions (615)
* `executor.future.MFuture`: keep CancelledError from dask instead of wrapping it within a MapcheteTaskFailed (615)
* `commands.index()`: add FlatGeobuf as index driver (616)
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