Make SSTableScanner always respect its bound

When SSTableScanner takes a DataRange, it doesn't fully respect the bounds provided as it always generate a Bounds object, thus potentially ending up including a key it should have excluded. It's currently compensated by in ColumnFamilyStore.getSequentialIterator but that is still an unexpected behavior and is such error prone. We should fix that and remove the compensation in ColumnFamilyStore.getSequentialIterator.