Enable to reuse already optimized layers without conversion
Enable to reuse already optimized layers without conversion

Currently, `ctr-remote i optimize` always converts all layers including already
optimized eStargz layers. However, sometimes we want to *reuse* these optimized
layers for saving resources consumed for these conversion.

This commit adds an option which enables to reuse already optimized eStargz
layers without conversion. With this option, a layer is reused when it is a
valid eStargz and no access occur to that layer during runing the workload.

In the future, we should remotely mount and lazily pull these eStargz layers
instead of downloading.

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

containerd/stargz-snapshotter,containerd/stargz-snapshotter,containerd/stargz-snapshotter,containerd/stargz-snapshotter