tor-proto: Make `Channel::wait_for_close` non-experimental

This changes the signature of Channel::wait_for_close and makes it non-experimental.

Follow-up from !2645 (merged).

The plan is to allow the channel manager to be notified when a channel closes.

---

Previously Channel::wait_for_close had a TODO to consider returning a "status indication instead of just ()", so this MR adds some status indication that we can expand later if we want to without requiring a backwards-incompatible change.

Old signature:

pub fn wait_for_close(&self) -> impl futures::Future<Output = ()> + Send + Sync + 'static {

Summary of new signature:

pub fn wait_for_close(
    &self,
) -> impl Future<Output = StdResult<CloseInfo, ClosedUnexpectedly>> + Send + Sync + 'static
{ ... }

#[derive(Clone, Debug)]
#[non_exhaustive]
pub struct CloseInfo;

#[derive(Clone, Debug, thiserror::Error)]
#[non_exhaustive]
pub enum ClosedUnexpectedly {
    #[error("channel reactor was dropped or panicked before completing")]
    ReactorDropped,
    #[error("channel reactor had an internal error")]
    ReactorError(Error),
}