cluster_bounding_box_finder: fix race condition

This fixes a race condition if two nodes call the service at the same
time. This produced an error when trying to call rospy.get_param(),
which is not thread-safe.

The error that occurred was:

    [INFO] [WallTime: 1390147189.562328] finding the bounding box for a point cluster
    [INFO] [WallTime: 1390147189.563293] finding the bounding box for a point cluster
    Traceback (most recent call last):
      File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospy/impl/tcpros_service.py", line 615, in _handle_request
    Traceback (most recent call last):
      File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospy/impl/tcpros_service.py", line 615, in _handle_request
        response = convert_return_to_response(self.handler(request), self.response_class)
      File "/opt/ros/fuerte/stacks/object_manipulation/object_manipulator/scripts/cluster_bounding_box_finder_server.py", line 75, in find_cluster_bounding_box_callback
        response = convert_return_to_response(self.handler(request), self.response_class)
      File "/opt/ros/fuerte/stacks/object_manipulation/object_manipulator/scripts/cluster_bounding_box_finder_server.py", line 75, in find_cluster_bounding_box_callback
        (box_pose, box_dims, error) = self.find_cluster_bounding_box(req)
      File "/opt/ros/fuerte/stacks/object_manipulation/object_manipulator/scripts/cluster_bounding_box_finder_server.py", line 105, in find_cluster_bounding_box
        (box_pose, box_dims, error) = self.find_cluster_bounding_box(req)
      File "/opt/ros/fuerte/stacks/object_manipulation/object_manipulator/scripts/cluster_bounding_box_finder_server.py", line 105, in find_cluster_bounding_box
        self.cbbf.find_object_frame_and_bounding_box(req.cluster)
      File "/opt/ros/fuerte/stacks/object_manipulation/object_manipulator/src/object_manipulator/cluster_bounding_box_finder.py", line 147, in find_object_frame_and_bounding_box
        self.cbbf.find_object_frame_and_bounding_box(req.cluster)
      File "/opt/ros/fuerte/stacks/object_manipulation/object_manipulator/src/object_manipulator/cluster_bounding_box_finder.py", line 147, in find_object_frame_and_bounding_box
        self.base_frame = rospy.get_param("~z_up_frame", point_cloud.header.frame_id)
        self.base_frame = rospy.get_param("~z_up_frame", point_cloud.header.frame_id)
      File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospy/client.py", line 452, in get_param
      File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospy/client.py", line 452, in get_param
        return _param_server[param_name] #MasterProxy does all the magic for us
        return _param_server[param_name] #MasterProxy does all the magic for us
      File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospy/msproxy.py", line 115, in __getitem__
      File "/opt/ros/fuerte/lib/python2.7/dist-packages/rospy/msproxy.py", line 115, in __getitem__
        code, msg, value = self.target.getParam(rospy.names.get_caller_id(), resolved_key)
        code, msg, value = self.target.getParam(rospy.names.get_caller_id(), resolved_key)
      File "/usr/lib/python2.7/xmlrpclib.py", line 1224, in __call__
      File "/usr/lib/python2.7/xmlrpclib.py", line 1224, in __call__
        return self.__send(self.__name, args)
      File "/usr/lib/python2.7/xmlrpclib.py", line 1578, in __request
        verbose=self.__verbose
      File "/usr/lib/python2.7/xmlrpclib.py", line 1264, in request
        return self.single_request(host, handler, request_body, verbose)
      File "/usr/lib/python2.7/xmlrpclib.py", line 1294, in single_request
        response = h.getresponse(buffering=True)
      File "/usr/lib/python2.7/httplib.py", line 1018, in getresponse
        return self.__send(self.__name, args)
      File "/usr/lib/python2.7/xmlrpclib.py", line 1578, in __request
        verbose=self.__verbose
      File "/usr/lib/python2.7/xmlrpclib.py", line 1264, in request
        return self.single_request(host, handler, request_body, verbose)
      File "/usr/lib/python2.7/xmlrpclib.py", line 1289, in single_request
        self.send_request(h, handler, request_body)
      File "/usr/lib/python2.7/xmlrpclib.py", line 1391, in send_request
        connection.putrequest("POST", handler, skip_accept_encoding=True)
      File "/usr/lib/python2.7/httplib.py", line 856, in putrequest
        raise ResponseNotReady()
    ResponseNotReady
    [ERROR] [WallTime: 1390147189.567050] Error processing request:
    None
        raise CannotSendRequest()
    CannotSendRequest
    [ERROR] [WallTime: 1390147189.568114] Error processing request:
    None