PERFORMANCE FIX: Use single database transaction for batch flush

Before: 1000 separate transactions (1000 commits = SLOW)
After: 1 transaction with 1000 writes (1 commit = FAST!)

Changes:
- Separate batch items by type before writing
- Open ONE transaction
- Write all headers, state diffs, and casms
- Commit ONCE
- Post-process compiled classes (class manager) and base layer blocks

This should reduce batch flush time from minutes to seconds!

Author: dean-starkware