DX-1573: raise error on parallel execution with asyncio.gather()

[yunusemreozdemir]

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[linear]

DX-1573 workflow-py detect and handle parallel steps

sancar said:

We had one concern about the current state.

Currently, parallel steps are not supported(postponed because it will take some more time to implement it).
But, that we can’t stop our users to try to do parallel runs. They can always skip waiting the promises returned by steps.

emre.ozdemir What is the behaviour right now? Can we detect that there is use of parallel steps. and fail the workflow(with printing warning on their environmets) ?
If yes, that could be enough to release as is.
Otherwise, if the workflow will hang or act unexpectedly, it feels unsafe to release.