Make _stats accept user-generated aggregates

Sometimes the user already has aggregate data that needs to be merged.
This patch supports that use case by allowing the user to emit a JSON
Object from the map phase containing 'sum', 'count', 'min', 'max', and
'sumsqr' keys.  Users can freely intermix raw values and precomputed
aggregates in a single view.

BugzID: 14286