persist metadata to skip remote reads on restart in DB mode

[wswsmao]

Fix: #2228

test case:

nerdctl pull --snapshotter stargz ghcr.io/stargz-containers/ubuntu:24.04-zstdchunked
nerdctl pull --snapshotter stargz ghcr.io/stargz-containers/python:3.13-zstdchunked

ls /var/lib/containerd-stargz-grpc/snapshotter/snapshots/
1  2  3  4  5  6  7  8  9


before:
time systemctl restart stargz-snapshotter.service
real    0m14.576s
user    0m0.014s
sys     0m0.004s

after:
time systemctl restart stargz-snapshotter.service 

real    0m4.039s
user    0m0.008s
sys     0m0.009s

[wswsmao]

@ktock