fix: Start listening after schema cache load

This change ensures PostgREST starts listening on a server socket only after it loaded the schema cache and is ready to handle requests. It is no longer going to return 503 errors during startup until the schema cache is loaded.

test/spec/SpecHelper.hs

@@ -160,7 +160,8 @@ baseCfg = let secret = encodeUtf8 "reallyreallyreallyreallyverysafe" in
   , configAdminServerUnixSocketMode = 432
   , configRoleSettings              = mempty
   , configRoleIsoLvl                = mempty
-  , configInternalSCQuerySleep      = Nothing
+  , configInternalSCQuerySleepFst   = Nothing
+  , configInternalSCQuerySleepSnd   = Nothing
   , configServerTimingEnabled       = True
   }