Use dlx.marc.Diff for record comparison in dlx-dl-sync

[jbukhari]

dlx-dl-sync currently implements similar logic and has a similar purpose to the dlx Diff class. We can use this tool to do the record comparisons, instead of reimplementing it in dlx-dl-sync. The Diff class has recent updates to do more robust checks, including comparing the field order of two records.

dag-hammarskjold-library/dlx#351