Before snapshotter per runtime, CRI only supports a global snapshotter. So a snapshot can be uniquely identified by `snapshot_key`. With snapshotter per runtime enabled, there may be multiple snapshotters used by CRI. So only (snapshotter_id, snapshot_key) can uniquely identify a snapshot. Also extends CRI/store/snapshot/Store to support multiple snapshotters. Signed-off-by: Jiang Liu <gerry@linux.alibaba.com>