nerdctl ps slows down and errors with 350+ containers

I'm running a load test on my kubernetes cluster using [ClusterLoader2](https://github.com/kubernetes/perf-tests/tree/master/clusterloader2), which just runs a bunch of `pause` containers on each node. When I have around 350 containers, the performance of `nerdctl ps` is affected compared to `ctr`:

```
$ time sudo /usr/local/bin/nerdctl --debug-full -n k8s.io ps
FATA[0020] container "4690dc5561d03a5c89453546a3c5d7c0a7ce7c3938cac1560cf358a2c6c040e9" in namespace "k8s.io": not found

real 0m20.374s
user 0m0.284s
sys 0m0.117s
$ time sudo /usr/local/bin/nerdctl --debug-full -n k8s.io container ls
FATA[0034] container "4a08c9bdba3dc064ad82fbd583f992d409dd8ee3346bfd413ea010cae1f43030" in namespace "k8s.io": not found

real 0m34.376s
user 0m0.284s
sys 0m0.132s
$ time sudo ctr -n k8s.io c ls | wc -l
354

real 0m0.126s
user 0m0.081s
sys 0m0.091s
```

Also notice that the command fails due to a container being removed while the command was running (which becomes more likely the longer the command takes).

I think the race condition is caused by `ps.go` calling `c.Spec` on each container after fetching the list of containers, meaning that if a container is removed before we can inspect it, the command will error. Could be fixed by skipping the removed container rather than erroring if the error is "not found": https://github.com/containerd/nerdctl/blob/cee3b6a4840db6b5dd4019ef343af7bf4ba5c940/cmd/nerdctl/ps.go#L118-L122

Not sure what to do about the performance issue though if we have to make `O(n)` requests to `Spec` each container. Maybe we could do some of those requests in parallel?