=con Update Aggregator pattern to use a mutable.LinkedHashMap for improved performance

This pull request removes the custom `WorkList` singly linked-list implementation and instead uses a `scala.collection.mutable.LinkedHashMap`. This should result in improved performance, especially for the `unexpect/remove` use case. Previously these methods could be O(N) in the worst case, but now with a `Map` items can be removed in constant time. Unit tests for the aggregator pattern still pass (and the now unnecessary `WorkList` test cases were removed).

@akara