Snapshot/Restore: restore of indices that are only partially available in the cluster

Fixes the issue with restoring of an index that had only some of its primary shards allocated before it was closed.

Fixes #8224
Snapshot/Restore: keep the last legacy checksums file at the end of restore

 This commit fixes the issue caused by restore process deleting all legacy checksum files at the end of restore process. Instead it keeps the latest version of the checksum intact. The issue manifests itself in losing checksum for all legacy files restored into post 1.3.0 cluster, which in turn causes unnecessary snapshotting of files that didn't change.

Fixes #8119