When data in a ds_row is updated, clear modified_data

When data has been successfully updated on the database, take the data in
ds_row->modified_data and stash it into it's componenet parts (index, cols, and
row) and then set it back to an empty map.
Tweaks to storeModified to make it comply to the spirit of ds_row

While you can easily add data into modified_data whose keys don't align with any
of the columns in cols, we don't need to push that data back into cols / rows.
DS is meant to represent a row of data and abusing it in that manner doesn't
need to be supported.
Cleared out extraneous variables
Merge pull request #7 from bfad/ds_row-storeModified

Store and clear ds_row->modified_data on update