CSHARP-1096: Remove dependency on XML DLLs.
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.
CR Changes.
change authenticate to use the correct cancellationToken.
Minor code review changes related to CancellationTokens.
shortened timeouts in tests to make the run faster.
removed unused variable.
added DropDatabaseAsync to MongoClient.
added DropCollectionAsync to IMongoDatabase.
added CreateCollectionAsync to IMongoDatabase.
added RenameCollectionAsync to IMongoDatabase.
added IMongoIndexManager and DropIndexAsync.
added GetIndexesAsync to IMongoIndexManager.
added CreateIndex operation.