File data sources: Causality region isolation

[leoyvens]

See #3072 for background on causality regions. The current implementation of offchain data sources tries to isolate them by not allowing them to read from the store at all and using an ephemeral entity cache. However onchain data sources are still able to read offchain entities. The quick and dirty way to fix this is to prevent reading any entity with causality region != 0 from the store.