[SPARK-2253] Aggregator: Disable partial aggregation automatically when reduction factor is low

[rxin]

TODOs

• Figure out the config option names
• Decide whether we want to create a new interface or just reuse the existing. If user applications use the existing combineValuesByKey with the assumption the output key will be unique, this could potentially break their applications (however we never really documented what the behavior of this function should be)
• Implement the same functionality for the external spilling case

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[mateiz]

A few comments on this:

• We probably can't break the existing combineByKey through a config setting. If people want to use this directly, they'll need to use another interface. Otherwise we can have combineByKey do this on the map side but not on the reduce side.
• Since spilling is on by default now, we should probably add an implementation for that too before merging.

[pwendell]

@rxin this issue has gone stale. Do you mind closing this issue?