Provide performance flag for MongoDB batch method to allow unordered operations

[DRK3]

MongoDB allows a BulkWrite call to be ordered or unordered. Right now all calls to BulkWrite are always ordered. For callers that don't need this, allowing them to specify that the call can be unordered can allow MongoDB to process the operations faster.

[DRK3]

For Amazon DocumentDB, doing a large number of upsert operations in a BulkWrite slow is very slow. They suggested that that we could try breaking up large upsert batch operations into multiple concurrent requests to realize better overall performance. This may be worth trying, but this type of strategy is only possible if we have a way to specify that operations are unordered in the first place.

Original message from DocumentDB support:

if cluster resources are not fully utilized and there are no side effects of concurrent updates, we recommend redistributing your batches across a larger number of concurrent client threads.