Add Cleanup to snapshot API

Cleanup is an optional method a snapshotter may implement.
Cleanup can be used to cleanup resources after a snapshot
has been removed. This function allows a snapshotter to defer
longer resource cleanup until after snapshot removals are
completed. Adding this to the API allows proxy snapshotters
to leverage this enhancement.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

snapshots/proxy/proxy.go

@@ -184,6 +184,13 @@ func (p *proxySnapshotter) Close() error {
 	return nil
 }
 
+func (p *proxySnapshotter) Cleanup(ctx context.Context) error {
+	_, err := p.client.Cleanup(ctx, &snapshotsapi.CleanupRequest{
+		Snapshotter: p.snapshotterName,
+	})
+	return errdefs.FromGRPC(err)
+}
+
 func toKind(kind snapshotsapi.Kind) snapshots.Kind {
 	if kind == snapshotsapi.KindActive {
 		return snapshots.KindActive