Safer Custom rootQueues

[jshier]

Issue Link 🔗

#3480

Goals ⚽

This PR makes Session's convenience init a bit safer by retargeting the provided rootQueue onto a serial queue with the same label before using it as the delegateQueue and passing it to the root initializer.

Unfortunately we can't do the same thing in the root initializer, as creating an additional layer of DispatchQueue once the delegateQueue has been created means that all of our dispatchPrecondition(condition: .onQueue(queue)) checks fail, as the delegate queue would no longer be in the same hierarchy with the retargeted rootQueue. However, we do still get some benefit here, as it would've prevent the bug reported in #3480.

Implementation Details 🚧

This PR simply creates a new serial DispatchQueue targeting the provided rootQueue to use as the actual rootQueue.

Testing Details 🔍

An additional test has been added which uses a concurrent queue as the rootQueue to demonstrate the safety of this approach.