CCD-470 Web TestEngine demo

Because combiners perform local aggregation of word counts, thereby allowing the mappers to
process input data faster.

Because combiners perform local aggregation of word counts, thereby reducing the number of mappers that need to run.

Because combiners perform local aggregation of word counts, and then transfer that data to reducers without writing the intermediate data to disk.

Because combiners perform local aggregation of word counts, thereby reducing the number of key-value pairs that need to be snuff let across the network to the reducers.