To allow the ability to have guaranteed-unique cursors, we make two
important updates:
* Add new updated_at_ms and created_at_ms columns that are in
millisecond precision
* Guarantee uniqueness -- if multiple items are inserted at the same
millisecond, bump the new one by one millisecond until it becomes unique
This lets us use single-number cursors for forwards and backwards paging
through resultsets and guarantee that the cursor is a fixed point to do
(timestamp > cursor) and get new items only.
This updated implementation is backwards-compatible since multiple
appservers can be running and won't handle the previous method well.
