Updates to PerRowSecondaryIndex don't use most current values

The way that updates to secondary indexes are performed using SecondaryIndexManager.Updater is flawed for PerRowSecondaryIndexes. Unlike PerColumnSecondaryIndexes, which only require the old & new values for a single column, the expectation is that a PerRow indexer can be given just a key which it will use to retrieve the entire row (or as many columns as it requires) and perform its indexing on those columns. As the indexes are updated before the memtable atomic swap occurs, a per-row indexer may only read the previous values for the row, not the new ones that are being written. In the case of an insert, there is no previous value and so nothing is added to the index.