Validate src_uris concurrently

[dmannarino]

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Other information

[jterry64]

As mentioned in our huddle, I think it'd solve the problem better to make the invalid sources check a background task that just sets the version to failed rather than doing it upfront. But also might require better error messaging in the version metadata.

Alternative workaround: have datapump just send a directory rather than all the source URIs.