settings: move service setting into services.js

this also introduces getJson/setJson

src/mail.js

@@ -781,7 +781,7 @@ async function getMailAuth() {
 async function restartMail() {
     if (process.env.BOX_ENV === 'test' && !process.env.TEST_CREATE_INFRA) return;
 
-    const servicesConfig = await settings.getServicesConfig();
+    const servicesConfig = await services.getConfig();
     const mailConfig = servicesConfig['mail'] || {};
 
     debug(`restartMail: restarting mail container with mailFqdn:${settings.mailFqdn()} mailDomain:${settings.mailDomain()}`);