Handling of pg_service file

[dvarrazzo]

If the service file contains some of the parameters we handle in a special way we won't make use of them. For instance:

• multiple hosts in host
• host -> hostaddr async resolution
• connect_timeout
• load_balance_hosts

After the refactoring that introduced psycopg.conninfo._get_param() there should be an obvious place where pg_service values can be handled consistently.