TaskVine: Fix serverless functionality

[tphung3]

Changing a task's state to READY inadvertently pushes that task to the list of ready tasks, which is not what we want if that task is a library task. This fixes the issue #3402 and adds test code.
PR #3409 should also be coordinated with this PR for FunctionCall API.

The formatting of results from a FunctionCall is a bit weird, as it is in the json format (see vine_example_function_call.py:49), which implies that results are json-serializable. Maybe cloudpickle is the better alternative here.

[dthain]

list_rotate was written that way to avoid incurring the cost of a free and then another malloc which are surprisingly expensive in the inner loop of the scheduler.

That said, if it is incorrect or could otherwise be simplified, feel free to propose an improvement.

[tphung3]

PR is updated to conform to PR #3409.

The issue of result formatting (where the returned value is a string in json format) is fine as long as:

• Users are aware that they have to do json.loads(t.output)['Result'] to get the result. -> More responsibility on users.
• Results must be json-serializable. For those that are not json-serializable, users must use an implicit result serialization into bytestreams within the submitted functions (e.g., calling dill.dumps(result) right before returning from function execution) as bytestreams are json-serializable. -> More responsibility on users.

[dthain]

Thanks for disentangling this.

While I am content to let the function results be json-encoded for now, I'm not thrilled with the idea that the user is responsible for decoding from json. It seems to me that taskvine should do it for the user. (And that frees us up to do some other encoding later.)

What do you think?

[tphung3]

This PR is RTM.