Trying to see how the "ScriptManager.Header buffer" assertion is failing. Either
`headerCallback` is being called multiple times, or the script is corrupt. By
adding a similar assertion in various places, we can hopefully narrow (a) what's
going on and (b) what code is involved.
Also, switched the BufferPool from DoublyLinkedList to SinglyLinkedList. Was
just reviewing this code (to see if the buffer could possibly become corrupt)
and realized this could be switched.