Profile the actual DB hit, not the query preparation

cc @Anahkiasen @julien-c for code review.

`findOne` is only a wrapper to a call to `find` which only hide the `MongoCursor` object from us.
This can be seen here: https://github.com/mongodb/mongo-php-driver/blob/master/collection.c#L873 and this is backed by this 20k SO answer: http://stackoverflow.com/a/7961190