Bump version 0.2.1 -> 0.3.0 (Cargo.toml + Cargo.lock), add the 0.3.0
CHANGELOG entry, and update the README's supported features and roadmap.

Highlights since v0.2.1:
- PostgreSQL multi-catalog support (#117, #120, #121, #124, #132)
- Row lineage / `rowid` virtual column (#115)
- Maintenance API: DROP TABLE, expire_snapshots, cleanup/orphan reclamation (#122, #123)
- Writer tuning: compression override, row-group caps, streaming writes (#126, #127, #128)
- Numerous schema-evolution and write-atomicity read/write fixes