Replace materialized views

[antiguru]

Implement a mechanism to replace materialized views with new definitions. This is a work-in-progress with many rough edges, and not a full implementation.

The PR adds the following syntax:

• To replace a materialized view, the user creates a replacement:
  CREATE REPLACEMENT mv_replacement FOR MATERIALIZED VIEW mv AS SELECT ...
• The user then observes hydration progress. At some point, they decide it wasn't a good change after all:
  DROP REPLACEMENT mv_replacement
• Or, they figure it's a good change and apply it:
  ALTER MATERIALIZED VIEW mv APPLY REPLACEMENT mv_replacement

Caveats

The implementation will come with some properties worth pointing out:

• Data at a specific time before the write frontier is sealed, and we cannot change it anymore. This means that constant materialized views cannot be changed at all, and refresh-every MVs only after the current write frontier, which is the next refresh moment.

Here's a rough overview of where we stand:

• Parse CREATE REPLACEMENT
• Parse DROP REPLCACEMENT
• Parse ALTER MATERIALIZED VIEW .. APPLY REPLACEMENT
• Per-object write mode in compute.
• Schema changes