Fix ALTER SYSTEM transaction and connection tracking issues

- Add query_no_return_autocommit() method for commands that cannot run
  inside a transaction block (like ALTER SYSTEM)
- Make admin_db connection lazy (created on first use) to avoid being
  captured in active_connections_before snapshot at sync startup
- Revert incorrect fix that refreshed connections after closing admin_db
  (was capturing massive sync connections)

This fixes both the 'ALTER SYSTEM cannot run inside a transaction block'
error and the connection check assertion failure.