Audit intra process comms for thread safety

At best implement tests which exercise the thread safety of the intra process manager class, but also audit the design considering the possible ways in which lighting may strike.

This is a follow on to https://github.com/ros2/ros2/issues/59

Related comments:
- https://github.com/ros2/rclcpp/pull/73#discussion_r37443374
- https://github.com/ros2/rclcpp/pull/73#discussion_r37456105
