Add ability to reserve WAL upon slot creation via replication protocol.

 Add ability to reserve WAL upon slot creation via replication protocol.
 
 Since 6fcd885 it is possible to immediately reserve WAL when creating a
 slot via pg_create_physical_replication_slot(). Extend the replication
 protocol to allow that as well.
 
 Although, in contrast to the SQL interface, it is possible to update the
 reserved location via the replication interface, it is still useful
 being able to reserve upon creation there. Otherwise the logic in
 ReplicationSlotReserveWal() has to be repeated in slot employing
 clients.
 
 Author: Michael Paquier
 Discussion: CAB7nPqT0Wc1W5mdYGeJ_wbutbwNN+3qgrFR64avXaQCiJMGaYA@mail.gmail.com