qgen: support DELETE queries

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

• Modify Query object to have an DELETE statement, and implement DeleteClause.
• Modify QueryProfile to choose columns to be upserted
• Modify QueryGenerator to generate DELETE statements
• Modify SqlWriter to write DELETE SQL in Impala
• Modify SqlWriter to write DELETE SQL in PostgresQL
• Hook in the same DB comparison after DELETE that is used for INSERT