Introduces the Task and TaskDispatcher interfaces (TaskDispatcher.h),
ThreadPoolTaskDispatcher implementation (ThreadPoolTaskDispatch.h), and
updates Session to include a TaskDispatcher instance that can be used to
run tasks.
TaskDispatcher's introduction is motivated by the need to handle calls
to JIT'd code initiated from the controller process: Incoming calls will
be wrapped in Tasks and dispatched. Session shutdown will wait on
TaskDispatcher shutdown, ensuring that all Tasks are run or destroyed
prior to the Session being destroyed.
