AggregatorFactory was generic over itself, but it doesn't appear we
use this functionality anywhere (e.g. to allow the super class
to declare arguments/return types generically for subclasses to
override). Most places use a wildcard constraint, and even when a
concrete type is specified it wasn't used.
But since AggFactories are widely used, this led to
the generic touching many pieces of code and making type signatures
fairly complex
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