Allow dbname in pg_basebackup/pg_receivewal connstring

Allow dbname in pg_basebackup/pg_receivewal connstring

As physical replication work at the cluster level and not database
level, any dbname in the connection string is ignored. Proxies and
middleware used in connecting to the cluster might however need to
know the dbname in order to make the correct routing decision for
the connection.

With this the startup packet will include the dbname parameter.

Author: Jelte Fennema-Nio <[email protected]>
Reviewed-by: Tristen Raab <[email protected]>
Reviewed-by: Jim Jones <[email protected]>
Discussion: https://postgr.es/m/CAGECzQTw-dZkVT_RELRzfWRzY714-VaTjoBATYfZq93R8C-auA@mail.gmail.com