class DeviceControl

Administrative control over an installed device on the network stack.

An instance of `DeviceControl` maps to an instance of

[`fuchsia.hardware.network/Session`]. All interfaces generated from a single

`DeviceControl` instance share the same `Session` and set of device buffers;

and are therefore subject to backpressure over the same pool of resources.

By the same measure, creating multiple `DeviceControl` instances attached to

the same underlying device causes data copies, because each `DeviceControl`

starts a new `Session`. For that reason, users should avoid creating

multiple `DeviceControl` instances for the same device and prefer

instantiating ports into interfaces from a single `DeviceControl` instance

per device.

This protocol encodes the underlying device's lifetime in both

directions; the device exists iff both ends of the protocol are open.

That is:

- Closing the client end causes the device to be removed, including all

interfaces created from it.

- Observing a closure of the server end indicates the device (and all

interfaces created from it) no longer exists.