qgen: implement SqlWriter methods for INSERT statements

Once IMPALA-4343 is finished, implement the SqlWriter methods for both Postgres 9.5+ and Impala for INSERT queries. This involves serializing the Pythonic object model representation of a query and turning it into syntactically valid SQL.

References:

• https://www.postgresql.org/docs/9.5/static/sql-insert.html
• http://www.cloudera.com/documentation/enterprise/latest/topics/impala_insert.html
• IMPALA-4353