Add computed columns to soda fountain upsert flow

- Factored out RowDataTranslator from RowDAO and separated out client JSON -> SoQLValue and SoQLValue -> DC JSON
- Computed columns field name must start with colon, this ensures they are treated like system columns in select statements (returned in SELECT :* but not SELECT *). There were a bunch of ways we could have implemented this but based on talks with Marc, Robert, Franklin this had the best tradeoffs for now.
- Tightened up some of the column validations in Soda Fountain (a) Currently 2 columns are allowed to have the same field_name, which creates ambiguity in SoQL SELECT statements. (b) Don't allow client to specify computed column values in an upsert/replace
So it's not valid if the column name already exists as a system column?