When working with mongodb documents where you do an insert of multiple documents at once and let mongodb create the `_id` field they can end up with the same timestamp. This makes it difficult in application code to compare ObjectId fields soley based on the timestamp.

I think we should add `Before(id)` and `After(id)` methods to `primitive.ObjectId` which takes both the timestamp and counter field into consideration when comparing ObjectId so that it matches the sort order comparisons done on the server.

Common existing code is `firstId.Timestamp().Before(secondID.Timestamp())` but this subtle breaks when doing find queries for many documents created at the same time and trying to iterator it without a capped collection. ie when you do something like `{_id:{$gt:lastIDSeen}}` it ends up not working due to these faulty ID comparisons.

These methods are also required because you cannot do greater than or less than comparisons on a byte array in go. Only the equality operator works.