This change converts the keys and posting lists to a
backwards-compatible format so that backups work accross versions of
Dgraph. The restore logic is also changed to convert the data back to
the internal Dgraph formats.

This PR changes the restore logic so that only the keys corresponding to
the predicates that were assigned to the group at the time of the last
backup are restored. This change is intended to properly deal with
predicate moves.

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