feat(lastN): Add ability to dynamically change lasN

JitsiConference.js

     this.rtc.pinEndpoint(participantId);
 };
 
+/**
+ * Selects a new value for "lastN". The requested amount of videos are going
+ * to be delivered after the value is in effect. Set to -1 for unlimited or
+ * all available videos.
+ * @param lastN the new number of videos the user would like to receive.
+ * @throws Error or RangeError if the given value is not a number or is smaller
+ * than -1.
+ */
+JitsiConference.prototype.setLastN = function(lastN) {
+    if (!Number.isInteger(lastN) && !Number.parseInt(lastN)) {
+        throw new Error('Invalid value for lastN: ' + lastN);
+    }
+    if (lastN < -1) {
+        throw new RangeError('lastN cannot be smaller than -1');
+    }
+    this.rtc.setLastN(lastN | 0);
+};
+
 /**
  * @return Array<JitsiParticipant> an array of all participants in this
  * conference.

modules/RTC/DataChannels.js

     });
 };
 
+/**
+ * Sends a "lastN value changed" message via the data channel.
+ * @param value {int} The new value for lastN. -1 means unlimited.
+ */
+DataChannels.prototype.sendSetLastNMessage = function (value) {
+    const jsonObject = {
+        colibriClass : 'LastNChangedEvent',
+        lastN : value
+    };
+
+    this.send(jsonObject);
+    logger.log('Channel lastN set to: ' + value);
+};
+
 module.exports = DataChannels;

modules/RTC/RTC.js

             throw new Error("Data channels support is disabled!");
         }
     }
+
+    /**
+     * Selects a new value for "lastN". The requested amount of videos are going
+     * to be delivered after the value is in effect. Set to -1 for unlimited or
+     * all available videos.
+     * @param value {int} the new value for lastN.
+     * @trows Error if there is no data channel created.
+     */
+    setLastN (value) {
+        if (this.dataChannels) {
+            this.dataChannels.sendSetLastNMessage(value);
+        } else {
+            throw new Error("Data channels support is disabled!");
+        }
+    }
 }