Validate yaml configuration with a schema

[erthalion]

Currently all the user-facing configuration is validated manually (sometimes on demand). So far it worked due to careful approach, but I think it's obvious that centralized declarative verification (using something like jsonschema) would be much less error prone and more convenient.

At the same time, since it potentially could be a breaking change to introduce such validation right away, it probably makes sense to report all the validation errors as warnings as the first step.

[redbaron]

Do you think that logical conflicts such as #920 is within a scope of this issue?

[erthalion]

Interesting point. Yes, I think this kind of configuration limitations also should be described and validated declarative using json schema (e.g. at some point have oneOf in the schema and enumerate valid combination of wal_level and logical replication slot). Of course @CyberDem0n is right, it's not an easy task to describe all the possible dependencies as this one, but we can try to start from something.