CSHARP-1055: Replace CheckElementNames and CheckUpdateDocument with more flexible IElementNameValidator(s).
CSHARP-1055: Code review changes.
CSHARP-1055: Removed UpdateValidator from operations and added CommandValidator to CommandOperationBase.
refactored some core write operations a bit to support a refactoring of the legacy bulk api in order to allow the development of the new high-level crud api.