Enable JSON in the create/insert commands generated by dbgen.

"schema.sql" now creates tables `jorders` and `jlineitem` with
all the non-indexed columns collapsed into one big JSON column.

The TPC-H benchmark program "dbgen" now takes an option "-j"
which causes it to generate INSERT queries referring to the jorders
and jlineitem tables and inserting well-formed JSON instead of
individual columns. Without the "-j" option, dbgen's behavior
is 100% unchanged.

This change should have zero observable effects in the demo's
behavior, except that from the console you'll be able to see
the two new empty tables created by "schema.sql".

Reviewed by: nikita