Construct a handle wrapping a shared cancellation flag. Created by a
ClientTransport when it opens the listen stream. onCancel, when
supplied, is invoked exactly once on the first cancel() (after the flag
is set) so a single-channel transport can emit its stdio
notifications/cancelled for the listen request id.
Construct a handle wrapping a shared cancellation flag. Created by a ClientTransport when it opens the listen stream. onCancel, when supplied, is invoked exactly once on the first cancel() (after the flag is set) so a single-channel transport can emit its stdio notifications/cancelled for the listen request id.