Possible `event_callback_` leak: `MediaStream` never deletes its event callback

[OvOhao]

Possible event_callback_ leak: MediaStream never deletes its event callback

I found a possible native heap leak in MediaStream. onMediaStreamEvent stores a heap
Nan::Callback in the event_callback_ member, but the member is never deleted — not on
re-registration and not in the destructor — so it leaks for every stream that registers
an event callback.

File: erizoAPI/MediaStream.cc

Functions: MediaStream::onMediaStreamEvent, MediaStream::closeEvents, ~MediaStream

NAN_METHOD(MediaStream::onMediaStreamEvent) {
  ...
  me->setMediaStreamEventListener(obj);
  obj->has_event_callback_ = true;
  obj->event_callback_ = new Nan::Callback(info[0].As<Function>());   // heap alloc into member
}

The destructor path never deletes it:

void MediaStream::closeEvents() {
  has_stats_callback_ = false;
  has_event_callback_ = false;
  ...                       // no delete of event_callback_
}

event_callback_ (a heap Nan::Callback holding a v8::Persistent to the JS function) is
never deleted anywhere in the file, so every MediaStream that called
onMediaStreamEvent leaks that callback when the stream is destroyed, and re-registering
leaks the previous one.

The sibling class does this correctly — WebRtcConnection::~WebRtcConnection calls
delete event_callback_;. MediaStream is missing the equivalent delete.

Suggested fix: delete event_callback_; (and stats_callback_) in closeEvents()/the
destructor, and delete the previous callback before overwriting the member on
re-registration.