Increased wait time between Mailgun events to 150ms

no issue

Reduce concurrency if the database is going a bit slower, until we have more permanent fix for this.

ghost/email-service/lib/email-event-processor.js

@@ -2,7 +2,7 @@ const {EmailDeliveredEvent, EmailOpenedEvent, EmailBouncedEvent, SpamComplaintEv
 
 async function waitForEvent() {
     return new Promise((resolve) => {
-        setTimeout(resolve, 100);
+        setTimeout(resolve, 150);
     });
 }