Change default values for backup and replication parameters

Change default values for backup and replication parameters

This changes the default values of the following parameters:

wal_level = replica
max_wal_senders = 10
max_replication_slots = 10

in order to make it possible to make a backup and set up simple
replication on the default settings, without requiring a system restart.

Discussion: https://postgr.es/m/CABUevEy4PR_EAvZEzsbF5s+V0eEvw7shJ2t-AUwbHOjT+yRb3A@mail.gmail.com

Reviewed by Peter Eisentraut. Benchmark help from Tomas Vondra.