DATAMONGO-912 - Consider custom mongo conversions in all stages of an aggregation pipeline.

We now consider custom mongo conversions in all stages of an aggregation pipeline. Previously we did this only for the first stage and returned object basically unmapped in later stages.
We now pass the root AggregationOperationContext on to nested ExposedFieldsAggregationOperationContexts so that those can delegate any mongo Mapping to the root context.