Performs the enhancement.
It is possible to invoke this method concurrently, but when doing so make sure
no two enhancement tasks are invoked on the same class in parallel: the
Enhancer implementations are not required to guard against this.
className - The name of the class whose bytecode is being enhanced.
originalBytes - The class's original (pre-enhancement) byte code
The enhanced bytecode. If the original bytes are not enhanced, null is returned.