CSHARP-1074: Reimplement legacy Aggregate methods using an operation in Core.
CSHARP-1074: Reimplement legacy AggregateExplain method on top of Core.
CSHARP-1100: Remove Command and ServerInstance properties from CommandResult.
removed timeout and refactored cancellation everywhere in Core except Operations.
removed timeout from operations and clarified the role of cancellation tokens in cursors and operations.