changefeed: fix schema backfill after initial scan checkpoint

[samiskin]

A changefeed that was restarted during a backfill with a checkpoint laid
down would neglect to clear the checkpoint information from aggregator
nodes, causing future backfills triggered by schema changes to reuse the
old checkpoint and skip spans.

This patch clears the kvFeed's checkpoint data upon scan completion

Release justification: low risk bug fix
Release note (bug fix): fix successive schema change backfills from
skipping spans that were checkpointed by an initial backfill that was
restarted

[cockroach-teamcity]

This change is 

[samiskin]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

---

error creating backport branch refs/heads/blathers/backport-release-21.2-77797: POST https://api.github.com/repos/cockroachlabs/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch 21.2.x failed. See errors above.

---

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is otan.}