Given the RequestServer created the request fd with socketpair() on
Windows and pipe2() on Unix, this abstraction avoids inlined ifdef soup
by hiding the details of how the AK::Stream and Core::Notifier are
created.
When the request is stopped, we clear its internal stream data. There is
a window where RequestServer may have sent an IPC message whose callback
will try to access that data in the time between the data being cleared
and RS receiving the stop signal. When this happens, just bail from IPC.
