util: Support to have shared local data on LocalPoolHandle

[nkzawa]

Motivation

Currently, there is no way to have shared thread-local data. It's required when data accessed from tasks is !Send.
This is useful for stateful applications like a game server, or when you want to have like per-thread cache.

Solution

Allow having shared local data. Also, add the spawn_pinned_at method which works the same with spawn_pinned except you can specify which worker thread the task is spawned.

I tried to make it a minimum, but it's a breaking change, unfortunately.

[Darksonn]

I will review this next time we make a breaking release of tokio-util.

[nkzawa]

Didn't know that but just found the std::thread_local! macro exists. In that case, seems only a method to spawn onto a specific thread is necessary, and it'll not be a breaking change any more. I will change like that if it's preferable.

EDIT:
Seems there are some disadvantages with thread_loca! like the behavior of destruction, and it cannot have generics.
The library https://crates.io/crates/thread_local requires the data to be Send

[Darksonn]

I'm going to close this because I don't think we want this. You can implement the feature yourself using Rc or thread_local!.

The ability to spawn on a specific thread will be added by #4680.