Some keys have unexpected size during DBWithTTL::Open()

### Expected behavior
All rows in our DB were inserted with keys of a fixed size. When calling `DBWithTTL::Open()`, all key comparisons should be on keys of the expected size.

### Actual behavior
During `DBWithTTL::Open()`, comparisons are being made on some rows that have keys that are a greater size than expected.

This breaks our custom Comparator which expects keys with a fixed number of bytes. We are not sure where the extra bytes are coming from.

### Steps to reproduce the behavior
Open TTL DB with custom comparator that throws exception if unexpected key size. Insert large volume of rows with fixed key size into TTL DB. Close and re-open the DB. During Open(), comparator will throw exception.