Many places in Active Support and Rails in general use `Thread.current#[]`
to store "request (or job) local data". This often cause problems with
`Enumerator` because it runs in a different fiber.
On the other hand, some places migrated to `Thread#thread_variable_get`
which cause issues with fiber based servers (`falcon`).
Based on this, I believe the isolation level should be an application
configuration.
For backward compatibility it could ship with `:fiber` isolation as a default
but longer term :thread would make more sense as it would work fine for
all deployment targets except falcon.
Ref: https://github.com/rails/rails/pull/38905
Ref: https://github.com/rails/rails/pull/39428
Ref: https://github.com/rails/rails/pull/34495
(and possibly many others)
