Major Description
The current Crunch-on-Spark mapside combiner uses a Multimap of key-value pairs to cache values for local aggregation. This is awesome, except it means that identical key-value outputs before a shuffle will only have one copy in the Multimap, which means that the aggregation counts may not be correct. We should fix it to use a proper Map<K, List<V>> to ensure that duplicate values are aggregated correctly.