refactor: remove dead code, process dict usage, fix shutdown

[Taure]

Summary

• Remove commented-out code blocks (requires_statement_description, portal_suspended, set_succeeded_or_within_failed_transaction).
• Thread query through loop state tuples instead of storing in process dictionary via put(query, Query) / get(query). The query is now carried in the parse_complete_with_params and parameter_description_with_params state tuples.
• Increase connection shutdown timeout from 100ms to 5000ms to allow graceful connection close on slow networks.

Test plan

• All 54 existing CT tests pass
• xref clean
• dialyzer clean

[tsloughter]

Thanks for the historical cruft cleanup!