bugfix: establish conference start time for parties that have been waiting

lib/tasks/conference.js

@@ -371,6 +371,7 @@ class Conference extends Task {
    */
   notifyStartConference(cs, opts) {
     this.logger.info({opts}, `Conference:notifyStartConference: conference ${this.confName} has now started`);
+    this.conferenceStartTime = new Date();
     this.emitter.emit('join', opts);
   }