Add support for aggregates (Yugo Nagata)

In this release, built-in count, sum, and avg are supported.

Add support for TRUNCATE on base tables (Yugo Nagata)

Add refresh_immv() function (Takuma Hoshiai, Yugo Nagata)

This is a function to refresh IMMV like REFRESH MATERIALIZED VIEW command. It has a boolean parameter corresponding to the WITH [NO] DATA option. When this flag is set false, the IMMV gets unpopulated, and immediate maintenance on the IMMV is disabled.

Fix error codes for invalid input to create_immv() (Yugo Nagata)