fix scanning over readless increments

The problem was in IncrementSummingScanner: the scan with batch=unlimited over single delta increment followed by another cell would miss out that delta increment and jump over rows.

Added tests on lower level in IncrementSummingScannerTest and on higher level for in OrderedTableTest. However (see https://issues.cask.co/browse/CDAP-970), keep in mind that the latter executes only against 96 hbase version.

Also, could be that there's potential issue when scanning with limit set (we don't do that, but likely do in the future to support common technique to avoid OOM of wide rows): created https://issues.cask.co/browse/CDAP-971. Not sure though - we'll have to dig later.