Make batch_size in kv.Scan effective for first Scan of iterator

[itaiad200]

Closes #7864

Tested manually. Requested the first entry in a prefix with tombstones.

1. AWS DDB, lakeFS running locally: 4162 leading tombstones, the fix reduced the latency from 1m40s to 2.5s.
2. Postgres & lakeFS running locally: 40K leading tombstones, the fix reduced the latency from 20s to 452ms.
3. Azure CosmoDB & lakeFS running locally: 931 leading tombstones, the fix reduced the latency from 43s to 40s (cosmosdb page size are harder to reduce after the first scan).

[github-actions]

E2E Test Results - DynamoDB Local - Local Block Adapter

[arielshaqed]

Neat! Thanks. And having all the tests makes this really easy to review, so kudos to all KV test writers throughout time!