[Feature Request] Do not set workflow completion until after all coroutines have settled in the task

[cretz]

Describe the solution you'd like

See https://github.com/temporalio/features/issues/481

[dandavison]

https://github.com/temporalio/sdk-core/pull/776 has been merged, and so this is unblocked. In sdk-python and sdk-dotnet that PR allows the https://github.com/temporalio/features/issues/481 requirement to be solved relatively easily, by changing lang so that they no longer drop post-terminal commands, as they had been doing.