When doing a bulk insert into a collection with a column index, we should also bulk insert keys into the column index itself. This can be done by generating all of the keys for the documents inserted, and inserting all of the keys for path 'a' together, then all of the keys for path 'b' together, etc etc.

To do this, we should change the column key generator so that it can generate keys for multiple documents, and then visit all of the keys for one path, then the next, and so on.