Snapshot/Restore: keep the last legacy checksums file at the end of rest...

...ore

 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
can we have a quick javadoc comment what this method does? Not related but I think we should maybe move all these legacy checksum related stuff in it's own inner class and only sync on that class instead of the directory etc.