add support for running in AWS autoscale cluster with graceful scale-in and standby

lib/session/session-tracker.js

@@ -44,6 +44,7 @@ class SessionTracker extends Emitter {
     assert(callSid);
     this.sessions.delete(callSid);
     this.logger.info(`SessionTracker:remove callSid ${callSid}, we have ${this.sessions.size} being tracked`);
+    if (0 === this.sessions.size) this.emit('idle');
   }
 
   /**