Check settings are the same between primary and secondaries

[Krysztophe]

In many cases the primary and a secondary must keep the same configuration, in case of a switchover.

In many cases people forget to propagate these changes. So a promoted primary will not archive, its work_mem will be different...

A new service could compare the pg_settings of a primary and a slave and raise a warning if there is a difference.

Optional --exclude option for some parameters that may differ (like cluster_name on Debian)

[Krysztophe]

Just wondering if this service should include pg_hba_file_rules (10+) or if it should be another service. IMHO, the latter.

[Krysztophe]

I forgot this and complained again in #283 , with more details, so I close this one.