=con Update Aggregator pattern to use a mutable.ArrayBuffer

This pull request removes the custom `WorkList` singly linked-list implementation and instead uses a [scala.collection.mutable.ArrayBuffer](https://github.com/scala/scala/blob/master/src/library/scala/collection/mutable/ArrayBuffer.scala). Unit tests for the aggregator pattern still pass. See below for benchmark information. The main benefit of going with `ArrayBuffer` is that we don't have to maintain our own custom singly linked list implementation in Akka. This may or may not be a big deal.

@akara