Bug Addressables - DeferredCompleteCallbacks concurrent race-condition bug

It leads to unpredictable results - events skipping, null reference exception, infinite loop or even memory damage. It can happen on failed WebRequest for example for now.

Test to prove: https://github.com/nick-dodonov/com...mmit/968f05c255121326c2eafa3ca6ade4e3733754d5

Fix: https://github.com/nick-dodonov/com...mmit/efe943f9e38cba8febbc2b7cc9c1e149c2693e88

 

Or possibly it's my fault - to think I can implement ResourceProvider in a real async manner.((
Just about everything in unity is main-threaded and AsyncOperation of WebRequest looks the same.

In such case the bug is better to transform to the "feature request" - to Assert (in debug only) public api methods that are not thread-safe.
* It'll give a better understanding of what is really allowed.
* It'll safe people starting to use Tasks without looking at SynchronizationContext and ContinueWith such public calls.