Allow manually remove invalid snapshots on restore

[vkuzniet]

Is your feature request related to a problem? Please describe.
On restart, soci-snapshotter-grpc doesn't start if one of the snapshots cannot restore. This makes impossible to manually remove images and snapshots (e.g. by using ctr).

Describe the solution you'd like
Cherrypick containerd/stargz-snapshotter#901

Describe alternatives you've considered

Additional context

[hanyuel]

This problem that this issue tries to fix is that if the snapshotter crashes, it won't be able to restart because the snapshotter won't be able to restore the remote snapshots (this will error out), so it will crash again and again.

I was not able to reproduce the problem by killing (SIGKILL-ing) the soci-snapshotter-grpc process when the container is running. I might have repro'd once... Here are the steps I did:

1. start soci-snapshott-grpc
2. soci image rpull $IMAGE
3. soci run $IMAGE
4. sudo kill -9 process-id-of-soci-snapshotter-grpc
5. ctr i rm $IMAGE
6. repeat step 1,2,3