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