nydusd failover and live upgrade in fuse dev

[imeoer]

• Failover: in the event of a nydusd crash or other failures, the nydusd process can be restarted from a snapshot. This will ensure that the container filesystem served by nydusd is still available.

• Live upgrade: nydusd can be upgraded without disrupting the container filesystem it serves. This is done by creating a snapshot of the running nydusd process, upgrading the binary, and then restoring the snapshot.

To achieve these capabilities, nydusd needs to store the following information in a persistent store:

• The fuse file descriptor for the fuse connection;
• The fuse state in fuse-backend-rs;

The nydus-snapshotter will be responsible for the workflow.

[imeoer]

cc @loheagn, we can sync the current implementation state here.

[loheagn]

#1487 and #1529 may fix this issue.