AMQP-334 Support Unwrapping Exceptions for Retry

With Spring AMQP, the exception thrown to the
RetryTemplate is a ListenerExecutionFailedException
with the business exception in the cause. This means
Exception categorization does not work.

Provide a retry policy/classifier that can examine
exception causes until a match is found.

If so configured, the retry policy should categorize
exceptions by traversing the cause if the current
exception is not itself categorized.