Changed ReadWriteConcern so that it always reads. Callers are now responsible for deciding whether to call it or not. A very small amount of extra code at the calling site but removes the subtle fact that ReadWriteConcern was sometimes a no-op.
CSHARP-797: Added BulkModify (stubbed out for now, server doesn't support batches with mixed operations, but we can emulate that by finding runs of each operation type and sending those as batches). Also added params versions of BulkModify methods to make calling them friendlier.