Use admin_db connection for ALTER SYSTEM commands

ALTER SYSTEM requires superuser privileges. Now hivemind keeps an admin
database connection from initialization and uses it for WAL safety
settings during massive sync.

- Added _admin_db class variable to store admin connection
- Added admin_db() and close_admin_db() methods
- WAL safety functions now use admin connection for ALTER SYSTEM
- Admin connection is closed after massive sync completes
- Gracefully skips optimization if admin connection isn't available