sharedMode snapshotter doesn't shutdown nydusd when handling os signal INT and TERM

[changweige]

As a e2e test case comment says:

          # After the snapshotter container is stopped, it seems that Nydusd doesn't umount it
          # so we need to umount it here, otherwise you cannot delete this directory. 
          # Frankly, I don't know why Nydusd didn't clean up these resources.
          sudo umount -f /var/lib/containerd-test/io.containerd.snapshotter.v1.nydus/mnt

The reason is that nydus-snapshotter always forks a nydusd when starts but exits leaving the nydusd unsignaled.

In fact, we can't just terminate sharedMode nydusd when handling SIGINT and SIGTERM since it can still serve container images while restarting nydus-snapshotter. But we call terminate it when nydusd when snapshotter is aware that no container image is being served.

[changweige]

will be addressed by #260