Moves all Session member variables dedicated to shutdown into a new
ShutdownInfo struct, and uses the presence / absence of this struct as
the flag to indicate that we've entered the "shutting down" state. This
simplifies the implementation of the shutdown process.
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.
An orc-rt Session contains a JIT'd program, managing resources and
communicating with a remote JIT-controller instance (expected to be an
orc::ExecutionSession, though this is not required).
