add update_manifest tool (#7815)

[harshil-goel]

This PR adds dgraph updatemanifest tool. This can be used to migrate the manifest of the backups taken on release/v21.03 or their parallels to the state after this change #7810

Problem

Solution

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[mangalaman93]

I think, we should remove this tool. We should make sure that later versions of Dgraph are backward compatible.