MetadataUpdater: provide a mechanism to edit the statistics of a column in a table (or a partition of a table)

Two motivations:

(1) Cost-based Optimizer (CBO) depends heavily on the statistics of a column in a table (or a partition of a table). If we would like to test whether CBO chooses the best plan under different statistics, it would be time consuming if we load the whole table and create the statistics from ground up.

(2) As database runs, the statistics of a column in a table (or a partition of a table) may change. We need a way or a mechanism to synchronize.

We propose the following command to achieve that:

ALTER TABLE table_name PARTITION partition_spec [COLUMN col_name] UPDATE STATISTICS col_statistics [COMMENT col_comment]