Move client IO task into a struct (oxidecomputer#1124)

This PR moves the client IO task from a function (with 9 arguments and an
`#[allow(clippy::too_many_arguments)] ` annotation) into a `struct
ClientIoTask`.

In addition, there are two logical changes:

- The rx task is moved into a `struct ClientRxTask` with an `impl Drop` that
  ensures that we abort that task if it did not stop on its own.  There were
  previously paths where we exited the IO task without aborting the rx task,
  e.g.
  [here](https://github.com/oxidecomputer/crucible/blob/f9fc36ba/upstairs/src/client.rs#L2606)
- Sending a message can now be interrupted by the `stop` oneshot or the rx task
  timeout.  This prevents a lock-up that @leftwo noticed, where we'd get stuck
  [here](https://github.com/oxidecomputer/crucible/blob/f9fc36ba/upstairs/src/client.rs#L2582)
  forever.