Move the QueryBatchSize field into the BackingStore

[jasagredo]

          Seems like this would be more appropriate as a field in the `BackingStore` data type instead.

Originally posted by @nfrisby in #1267 (comment)

Perhaps we can remove this field from the LedgerDB top level API and put it into the BackingStore and capture it in the LedgerBackingStoreValueHandle.

Care has to be taken because we adapt the actual number of requested values based on the current diffs in the V1 changelog, so we might not be able to directly remove it from the RangeQuery datatype.

[jasagredo]

Also the V2 forker uses Maybe Int while it could also use QueryBatchSize. See #1267 (comment)

[jasagredo]

This was implemented in #1304