Future returned by the thread-safe control operations on Server and Client.

Unlike other pw_rpc2 futures, which may only be used on the dispatcher thread, a ControlFuture may be created, moved, or dropped on any thread. Dropping a RegisterListener() future before it resolves withdraws the registration; Close() is already under way when it returns and continues even if its future is dropped.

Resolves to an RpcStatus. The codes each operation can return are documented on the Server or Client function that returns the future.