Prevent pg_enable/disable_data_checksums() on standby

Prevent pg_enable/disable_data_checksums() on standby

These functions missed a RecoveryInProgress() check, allowing them to
be called on a hot standby.  Enabling, or disabling, checksums on the
standby only would cause the cluster to get out of sync and replaying
checksum transitions to fail.

Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAHg+QDfRk4-S7DMmdbXJnQ-xF=sUpMAKuh8b83ObLqYVKx5QLA@mail.gmail.com