DRILL-1869: CPP client - Handle nullable varchar columns with all nulls (fixed slicedByteBuf to handle zero length buffers). Also handle case where server sends a record batch where vector value counts  are zero.
DRILL-1848 Unsupported operation exception
DRILL-1849: Make parquet readers return nullable int instead of nullable bit for unknown columns.
DRILL-1871: Compressed JSON read support.
DRILL-1886: Project on subquery containing a union all causes NPE