Use strings instead of numbers for dynamic event names / callback ids

Let me preface by saying I understand if you aren't keen on supporting 0.9 just yet. Though I'm trying to upgrade an application to 0.9 and the first error I'm getting is at `mongodb/lib/mongodb/connection/base.js:99`.

This is due to a current (in 0.9) breaking api change that throws given anything but strings for event names (https://github.com/joyent/node/blob/master/lib/events.js#L172). I think it would be fine to just change things like this:

     this._callBackStore.once(command.getRequestId(), callback);

to this

    this._callBackStore.once(command.getRequestId().toString(), callback);

I can go through and make the fix. Let me know if you have any thoughts.