Overflow record filter should handle records with duplicate key

NXP-26691 introduces a record filter that persists big value into external storage (a key-value store or a transient store).

The key used for the storage is based on the record key.

This creates a conflict if different records use the same key. At best the consumer will read the latest write or it will not find the value because of a race condition in the storage (NXP-27292).

To prevent this conflict the key used by the storage must be unique.

This can be easily done by combining the record's key and the record's watermark.