PostgreSQL "better" materialized views

PostgreSQL's materialized views don't get updated on-demand. Users need to refresh a materialized view, and the [refresh command](http://www.postgresql.org/docs/current/static/sql-refreshmaterializedview.html) discards old contents and completely replaces the contents of a materialized view.

Commercial databases incrementally update a materialized view's contents -- this is particularly helpful for aggregate queries. This task involves improving PostgreSQL's materialized views for incremental updates.

Start with the PostgreSQL REFRESH MATERIALIZED VIEW documentation linked in the issue and compare its full-replacement behavior with the requested incremental updates. Identify the Citus entry points and tests for materialized-view refresh behavior; done means supporting incremental updates, including for aggregate queries, without discarding all existing contents.

Written by the indexing model from the issue text.