Using `PkvStore` from `IoTaskPool`

[bonsairobo]

The pattern of keeping PkvStore as a resource is not compatible with offloading usage onto a task pool, because PkvStore cannot be cloned.

Is the intended usage to create a new PkvStore for tasks?

I don't think wrapping PkvStore in Arc<RwLock<_>> is a good solution because most (all?) storage backends do not require this extra layer of synchronization.

It seems like it would also be possible to change the StoreImpl trait to use &self instead of &mut self and rely on the interior mutability/synchronization of the database implementations. Then we could actually have a lazy-initialized static global database handle which can be used from anywhere. Or we could only wrap PkvStore in Arc and clone it out of the resource for tasks.