fix possible concurrency in is_threading

[lmiq]

This fixes #317

I think the issue is a concurrency that may occur in the is_threading function.

I'm not sure if that function is necessary at all. The issue can be removed by always using the lock, and probably in serial runs the cost of that is irrelevant.

But maybe the bug reflects something deeper, so this might be the wrong fix.

(there is one test failing, but it is not related to this change)

[MarcMush]

Ref #232

[MarcMush]

As a workaround, you can imitate #298

[lmiq]

As a workaround, you can imitate #298

Ok, but this will break possibly many codes in Julia 1.11. A fix is now urgent. I would just do the safe strategy and just always lock, and remove that. Also since tasks can migrate, I have the impression that using threadid() to detect parallelism is not a good idea anyway.

[lmiq]

Closed because other PRs are dealing with this.

[MarcMush]

I agree the current code is bad and needs to be fixed
Is there a performance hit? If it is non-negligible we can imagine a keyword like thread-safe that defaults to true