=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](https://github.com/scala/scala/blob/master/src/library/scala/collection/mutable/LinkedHashMap.scala). 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 [LinkedHashMap](https://github.com/scala/scala/blob/master/src/library/scala/collection/mutable/LinkedHashMap.scala) items can be removed in effectively constant time. Unit tests for the aggregator pattern still pass (and the now unnecessary `WorkList` test cases were removed).

@akara