Implement very basic views using DBSP
This is just the bare minimum that I needed to convince myself that this
approach will work. The only views that we support are slices of the
main table: no aggregations, no joins, no projections.
* drop view is implemented.
* view population is implemented.
* deletes, inserts and updates are implemented.
much like indexes before, a flag must be passed to enable views.
Closes#2530
6e8d1a5c7d