Fixing old flapper (concurrent changes to cache are unsafe)

The say SObjectType is written - it's not safe running updates concurrently. Race conditions can and will happen causing the tests to flap.
The update with (describe,_) and (_, metadata) would cross and you would end up with (describe, _) and (metadata, ) instead of (describe, metadata). 
In the browser, it wouldn't really happen since everything is really happening on a single thread.

Maybe there is a way to make SObjectType thread-safe but for now, I would recommend we tell developers to chain their updates instead of having them run concurrently.