Fix disposal of marshalable object proxies

[matteo-prosperi]

This addresses #1087

[AArnott]

This looks good to me. Is the theory of the bug then that DisposeAsync was being called concurrently?

[matteo-prosperi]

@AArnott,
the issue was easily reproducible with the sample provided in #1087.

The sample has a loop of the following:

        await using var stream = new NamedPipeClientStream(".", "TestPipe", PipeDirection.InOut, PipeOptions.Asynchronous);
        await stream.ConnectAsync().ConfigureAwait(false);

        using var jsonRpc = JsonRpc.Attach(stream);
        var serverProxy = jsonRpc.Attach<IServer>();

        using var myService = await serverProxy.GetMyService().ConfigureAwait(false);
        await myService.DoSomething().ConfigureAwait(false);

Which results in frequent exceptions on the following line, due to the collection having been modified concurrently with the iteration

foreach (object target in this.localTargetObjectsToDispose)

I verified that the issue doesn't repro anymore after my change.

I believe that the cause of the concurrent change of the collection is RevertAddLocalRpcTarget.Dispose().