Support row scans of sparsely-populated rows.

Every fully-qualified column in a user's data request becomes a separate
SELECT statement that goes to Cassandra (this may change).  The
KijiRowScanner therefore gets back a separate Row iterator for each
fuly-qualified Kiji column.  Every Kiji row may not necessarily have
data for every fully-qualified column in the data request.  Therefore,
the Cassandra Row iterators that the Kiji row scanner gets may not
contain data for every Kiji EntityId.

This commit contains code to handle this case and figure out how to
stitch the data together across multiple Row iterators in the row
scanner.