Allow passing promises more flexibly

[adamnovak]

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In #3009 a user ran into a problem with the promise system; it seems like the wanted to so something that ought to be allowed to work (because there were no actual dependencies on promised data before it would have been ready), but they weren't allowed to because they way they wanted to do it would have required promises to be pickled and unpickled before being fulfilled.

We should try and make the Python API promise system more flexible, so that we can allow repeated pickling and unpickling of promises, as long as they aren't actually used before being resolved.

┆Issue is synchronized with this Jira Story ┆Issue Number: TOIL-1473

[unito-bot]

➤ Lon Blauvelt commented:

Details should be added to this. Not entirely clear how this should be added. This is also low priority, as we recommend using CWL or WDL over the python API.