SolarNetwork runs datum import jobs in parallel, going from oldest to newest, but it can happen that multiple imports are queued that, when executed in parallel, cause a deadlock because they overwrite the same datum.

A solution to prevent this is to add a "barrier" key to the import configuration. If provided, SolarNet would guarantee that only one import per key happened at once. They actual barrier key value used internally must be a combination of the import account's user ID + provided barrier key, so different users can provide the same key value without impacting each other.