Enable refresh materialized views with atomic swap

[mikicz]

Add an option to refresh materialized views with an atomic swap. When the refresh starts, it would create a new materialized view (with some temp name), and when it is created, it would run an atomic deletion of the old materialized view, and renaming of the new view to the correct name.

This could reduce the number of vacuums PostgreSQL is doing. It may also allow the view to be created without using the concurrent flag, which should be faster.

[radusuciu]

This would be a great feature, especially for larger materialized views.