Don't explicitly keep track of _activeListenerCount in the Aphlict server.

Summary: The `_activeListenerCount` variable is overkill, we should be able to achieve the same result using `Object.keys(this._listeners).length`.

Test Plan:
Mucked around in a NodeJS shell.

```lang=js
> Object.keys({}).length
0
> Object.keys({foo: 'bar'}).length
1
> Object.keys({1: 'foo', 2: 'bar'}).length
2
```

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9554

support/aphlict/server/lib/AphlictListenerList.js

@@ -9,14 +9,12 @@ JX.install('AphlictListenerList', {
   members: {
     _listeners: null,
     _nextID: 0,
-    _activeListenerCount: 0,
     _totalListenerCount: 0,
 
     addListener: function(socket) {
       var listener = new JX.AphlictListener(this._generateNextID(), socket);
 
       this._listeners[listener.getID()] = listener;
-      this._activeListenerCount++;
       this._totalListenerCount++;
 
       return listener;
@@ -26,7 +24,6 @@ JX.install('AphlictListenerList', {
       var id = listener.getID();
       if (id in this._listeners) {
         delete this._listeners[id];
-        this._activeListenerCount--;
       }
     },
 
@@ -42,7 +39,7 @@ JX.install('AphlictListenerList', {
     },
 
     getActiveListenerCount: function() {
-      return this._activeListenerCount;
+      return Object.keys(this._listeners).length;
     },
 
     getTotalListenerCount: function() {