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.
