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
Hmm... deletes is not an Iterator, because it is mapped from rows, which is a `Seq`.  So what is really going on here I wonder.   Is `deletes` automatically converted to an Iterator?

I think this would be clearer, if, instead of converting `upsertsWithComputed` to an Iterator in line 73, we leave it as a Seq, then do this here:

    (upsertsWithComputed ++ deletes).toIterator