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