The request.spec GUC contains the schema cache structure in json.
It's only available when the root endpoint(/) is requested and when
db-root-spec is not empty.
Also correct db-root-spec to accept a schema.
Now that PgVersion is not part of DbStructure, Config is a more apt
module for it.
Also rename getDbStructure to queryDbStructure. AppState also had a
getDbStructure function for a record field.
