CSHARP-798: Execute AggregateQuery immediately if $out is used. Remove FirstBatch property from AggregateResult and overload ResultDocuments to also hold the first batch when using a cursor. Add new RunAggregateCommand helper method to MongoCollection and remove overload of Aggregate command that takes an options parameter (so if you want to use a cursor, which requires options, you have to use the new AggregateQuery method).