Closing many stream servers can crash on windows
Running this branch (example included): https://github.com/status-im/nim-chronos/commit/bb121356bf97d5358aef6e0be813bcf5b88db5a5 on windows, results in:
```
[...]
Accept continuation
closed
Accept continuation
Accept continuation
[...]
```
It effectively means that the accept continuation can be called after `closeWait` finishes.

Before https://github.com/status-im/nim-chronos/pull/309, it started to happen with only 10 servers, now it needs ~190.
Since `closeWait` has finished, the user may have deleted his references to the `StreamServer`, like we do in libp2p:
```nim
    for server in self.servers:
      server.stop()
      toWait.add(server.closeWait())

    await allFutures(toWait)

    self.servers = @[]
```
So when the accept continuation is called, the GC may have already cleaned the `StreamServer`, which results in segfault.

I also suspect that if the dispatcher is more busy, it can happen with far less servers

We only hit this on windows, though I didn't spend time trying to replicate on linux