Revisit datatransfer event handling in retrieval

[elijaharita]

I've been noticing some retrievals finishing instantly without error, without actually ever receiving any data. I think some important events from datatransfer are not being handled properly in some cases.

It's a little confusing to me which datatransfer events are finalizing events, and which ones are repeated / non-final, and even if multiple "finalizing" events can happen in the same run. filclient should properly handle all exit paths from datatransfer but this is not happening right now.

@hannahhoward some input from you on this would be appreciated!! would you mind looking over the datatransfer callback in the retrieval function?

[corinne-antonia]

@elijaharita Did @hannahhoward get back to you?