Fix #714: Improve handling of observable notifications at page boundaries

These changes introduce logic to create and maintain overlap of
pages of query results, in order to counteract problems where
dojo/store/Observable drops items at page boundaries.
Fix improperly-initialized grids in unit tests
CHANGES.md: Note observable item dropping fix
CHANGES.md: Note resolution of issue showing items added at end

This was resolved during the work on overlapping observable results.
More robust check before eliminating observers

Check to ensure all rows have really been removed before eliminating
observers. Also provide better tolerance of missing rows.