Consider adding row delete markers

Since HBase does not currently provide a way to undo delete operations, Tephra currently implements deletes by writing a special delete marker for each column. In the case of a row delete, this means that we have to read the entire row being deleted, then issue a separate delete marker for each column in the row. In the case of a wide row with many columns, this can be very inefficient.

We should consider support row delete markers:

• for a row delete, we would record a special column family delete marker for each column family in the row
• the delete marker would be stored in a special reserved column name. Any attempted puts to the same column name would be rejected.
• we would have to ensure the delete marker sorts prior to any other columns in the column family
• on any get or scan operation, we would have to add this column to the requested columns for each column family being used
• if the delete marker exists, seek to the next row