Memory Leak in snapshotter::subscribe()

[arneboe]

This bug was already present and pointed out in the old PR (ros/ros_comm#1414).

As far as I can see it is still present in this version.

Copy&paste from old PR:
There is a memory leak in Snapshotter::subscribe.
You are binding topicCB using boost::bind with queue as additional parameter.
Therefore the subscriber holds a shared_ptr to the queue.
Storing the subscriber inside the queue (queue->setSubscriber(sub);) essentially makes the queue hold a shared_ptr to itself and thus it will never be deleted.
Never deleting the subscriber will in turn stop a whole bunch of ros internals from getting shutdown properly.

A possible fix (albeit not a very nice one) is.

Snapshotter::~Snapshotter()
{
    for (std::pair<const std::string, boost::shared_ptr<MessageQueue>>& buffer : buffers_) 
    {
      buffer.second->sub_->shutdown();
    }
}