Executor: force Function Executor RunTask concurrency=1

[eabatalov]

Previously before in-Process Function Executor was introduced max Function concurrency in Executor was 1. We bring the previous behavior here but enforce the concurrency of 1 per Function Executor (not for the whole Executor). It's better this way cause it's hard for customers to reason about implications of running multiple concurrent tasks per Function Executor.

Also added tests that verify task concurrency for the same and different functions.

We'll revise this concurrency policy in the future if we allow customers to configure their functions' concurrency explicitly via function attributes.

I also did a big refactoring which results in task policy implementation simplification. This is how the current task policies are implemented now:

await state.wait_running_tasks_less(1)

if state.function_id_with_version != _function_id_with_version(task):
    await state.destroy_function_executor()
    state.function_id_with_version = _function_id_with_version(task)

There also many other refactorings in this PR that help to implement the upcoming features in Executor and simplify the code base.

Testing:

make check
make test

Contribution Checklist

• If the python-sdk was changed, please run make fmt in python-sdk/.
• [n/a] If the server was changed, please run make fmt in server/.
• Make sure all PR Checks are passing.