TAJO-1067: INSERT OVERWRITE INTO should not remove all partitions.

I added new session variable for avoiding remove existing partition directories. Its name is COLUMN_PARITION_REMOVE_ALL_PARTITIONS. Its default value is false. Thus, if you run INSERT OVERWRITE statement with column partitioned table, tajo doesn't remove existing partition directories. Tajo just remove partition directories which is equals to staging directories.

And this patch has a lack that is related to TAJO-744. You left the comment for the lack at TAJO-1067.
In my view, ```isLeafDirectory()``` would be more intuitive name than ```!hasDirectory()```.