Index row impl

Create a new `FDBIndexRow`, with updates to the abstract `IndexRow` implementation. This new implementation allows pushing the `PersisititIndexRow[Buffer]` implementation details down into the new class, thus hiding (some) of their Persistity goodness. 

This branch addresses the read side of IndexRow. The Index write process still relies upon the PersistitIndexRowBuffer to write index rows, this will be address separately.