qgen: add UPSERT support

After all the INSERT upfront work, UPSERT should be relatively straightforward.

• Modify Query object to have an UPSERT statement, and implement UpsertClause.
• Modify QueryProfile to choose columns to be upserted
• Modify QueryGenerator to generate UPSERT statements
• Modify SqlWriter to write UPSERT SQL in Impala
• Modify SqlWriter to write UPSERT SQL in PostgresQL. Note this is really “INSERT ... ON CONFLICT DO UPDATE”.
• Hook in the same DB comparison after UPSERT that is used for INSERT