task: Add demo that converts a changefeed into a ledger

[Bob Vawter]

It would be an interesting experiment to write a user-defined apply function that adds the source MVCC time as a column, combined with the existing soft-delete demo. This would effectively convert the changefeed into an append-only ledger, which could be used for hot/cold/auditing use cases.

X-Ref: #705 for a userscript API to load records from the target and/or #747 for a cas: always behavior to do this with a merge function instead of an apply function.