Pass chained layer digests to snapshotter for parallel snapshot preparation
Pass chained layer digests to snapshotter for parallel snapshot preparation

Currently, CRI plugin passes each layer digest to remote snapshotters
sequentially, which leads to sequential snapshots preparation. But it costs
extra time especially for remote snapshotters which need to connect to the
remote backend store (e.g. registries) for checking the snapshot existence on
each preparation.

This commit solves this problem by introducing new label
`containerd.io/snapshot/cri.chain` for passing all layer digests in an image to
snapshotters and by allowing them to prepare these snapshots in parallel, which
leads to speed up the preparation.

Signed-off-by: Kohei Tokunaga <72a466a7d91579e23b8af87d6766ee221241ce87@gmail.com>

thaJeztah/containerd,estesp/containerd,containerd/containerd,vdemeester/containerd,dmcgowan/containerd,mikebrow/containerd,thaJeztah/containerd,mikebrow/containerd,dmcgowan/containerd,vdemeester/containerd,estesp/containerd,containerd/containerd