nydus-image: support chunk level deduplication between local images

Add the dedup option to nydus-image to support block level deduplication
between local images. A local database cas.db was introduced, which records
all chunks saved locally. Then We can redirect the chunks to those that already
exist locally to avoid duplicate downloads.

There is still some work needs to be done, including:
1. Insert the on-demand downloaded chunk into the database to ensure that
the database stores locally existing chunks.
2. Snapshotter's GC mechanism with the reference relationship of bootstrap
after deduplication.

Signed-off-by: Huang Jianan <[email protected]>
Signed-off-by: xwb1136021767 <[email protected]>