consider task dependencies on shutdown

[asteven]

The "credentials retriever" should not be stopped before other tasks, that depend on valid credentials, have been stopped.

closes: #1033

[nolar]

A note mainly for self:

I see the stated problem — indeed, a problem. However, this solution is problematic too. spawn_tasks() and run_tasks() are a part of the public interface of Kopf (kopf/__init__.py). This change alters the declared signatures, thus breaks the backwards compatibility, therefore requires a semantic v2 release, which is not planned.

I need to try some other approaches prioritising or linking tasks.

1. One way that I see is inheriting from asyncio.Task and adding a few fields with priorities/dependencies. However, the Task is a C-level unit, not pure Python, so some optimizations can be lost by inheriting it (as with dicts/ints/strs/etc).

2. Another way is to inject hidden attributes, but this behaviour is not documented, so it is not clear how it behaves with optmized event loops (e.g., uvloop), maybe with slots.

These are the thoughts to the moment. Maybe I will come up with some other ideas later.

[nolar]

I am closing this PR in favour of the newer one — which seemingly solves the same problem the same way (more or less), but without breaking changes, and using the StdLib's functionality (no hacks).

• Terminate operator tasks sequentially in groups #1282