Migration corner-case: what if the schema changed upstream in Hive. `SYNC` is supposed to be idempotent and intended to run multiple times.

[nfx]

          What if the schema changed upstream in Hive. Sync is supposed to be idempotent and intended to run multiple times.

Originally posted by @zpappa in #295 (comment)

[pohlposition]

In this scenario, the changed tables would be represented as new tables in the mapping table; and then the normal process [TBD] would continue.

[nfx]

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