capnp-rpc: Add disconnect_background to Disconnector

[griff]

This adds a public disconnect_background function to Disconnector so that you can signal it to disconnect but not have to wait for it to do so.

I usually run the RpcSystem with spawn_local and would like to be able to just signal that it should shut down in like a Drop implementation for my Client.

It would also make my graceful shutdown listener for the server slightly less clunky since that needs to do this:

if this.cancelled_guard.is_some() {
    match (
        this.rpc_system.as_mut().poll(cx)?,
        this.disconnector.as_mut().poll(cx)?,
    ) {
        (task::Poll::Ready(_), task::Poll::Ready(_)) => task::Poll::Ready(Ok(())),
        _ => task::Poll::Pending,
    }
} else {
    this.rpc_system.poll(cx)
}

While with this PR it would look like this:

if this.cancelled_guard.is_some() {
    this.disconnector.disconnect_background();
}
this.rpc_system.poll(cx)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.65%. Comparing base (ab342b3) to head (6776bd7).
⚠️ Report is 188 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
- Coverage   51.64%   50.65%   -1.00%     
==========================================
  Files          69       70       +1     
  Lines       33735    32295    -1440     
==========================================
- Hits        17422    16358    -1064     
+ Misses      16313    15937     -376     

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