This CL mainly made the following modifications: 1. Reorganized SegmentV2 upgrade document. 2. When the variable `use_v2_rollup` is set to true, the base rollup in v2 format is forcibly queried for verifying the data. 3. Fix a problem that there is no persistent storage format information in the schema change operation that performs v2 conversion. 4. Allow users to directly create v2 format tables.