Restrict db settings to current db and global

Global settings are overriden by database specific settings if they
share common ones.

test/io-tests/configs/expected/no-defaults-with-db.config

@@ -1,7 +1,7 @@
 db-anon-role = "postgrest_test_anonymous"
 db-channel = "postgrest"
 db-channel-enabled = true
-db-extra-search-path = "public,extensions"
+db-extra-search-path = "public,extensions,private"
 db-max-rows = 1000
 db-pool = 1
 db-pool-timeout = 100
@@ -14,7 +14,7 @@ db-tx-end = "commit-allow-override"
 db-uri = "<REPLACED_WITH_DB_URI>"
 jwt-aud = "https://example.org"
 jwt-role-claim-key = ".\"a\".\"role\""
-jwt-secret = "REALLYREALLYREALLYREALLYVERYSAFE"
+jwt-secret = "OVERRIDEREALLYREALLYREALLYREALLYVERYSAFE"
 jwt-secret-is-base64 = true
 log-level = "info"
 openapi-server-proxy-uri = "https://example.org/api"