Since a view is defined by an aggregation pipeline, we convert any count, distinct, or find command request to an equivalent aggregation command.

For the count command, this is accomplished with CountRequest::asAggregationCommand(). Currently this returns a BSONObj, which we almost always parse immediately into an AggregationRequest. For performance and simplicity, we should change these methods to return a StatusWith<AggregationRequest> directly.

Also, if the caller always uses uassertStatusOK on the result, we can further simplify by returning AggregationRequest directly, and throwing an exception in the error case.