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
Yuck.  I know you didn't write this, but I wonder why we mixed using `java.util.HashMap` with scala Maps?

If we replaced columnInfos with a scala Map, the whole `match { case null =>` stuff can be replaced with `columnInfos.getOrElseUpdate(rawColumnName, { ... }`.  Except for the case when it calls `columnInfos.clear()`....