Use throttle instead of debounce and use `getNextPage` instead of setting page size

Changes:

1. Scrolling is now throttled so it doesn't hit the server too quickly. Previously it was debounced and the scrolling experience was too slow.
2. Throttling wait time is configurable by subclassing.
3. Removed `rowPerFetch` and substituted that with `PageableCollection#state.pageSize`.
4. Use `getNextPage` instead of `setPageSize`. backgrid-infinator now assumes the collection is in infinite mode.
5. Juggled around the paths in `index.html` for ease of merging with mainline in the future.

Please review these changes and see if they make sense.

Thanks again for the idea in this extension.