Catalog generator should print warnings if there are sequential scans

[apavlo]

I'm worried that our query planner for TPC-C isn't always generating the best plan. We should add an option to hstore-prepare that prints out the name of queries that contain sequential scans.

[xinjiacs]

It's better to have a planner tool to guarantee the plans are not changed from the baselines. It may help to identify more unexpected plan changes.

FYI, VoltDB has a planner tool to do this kind of job. https://github.com/VoltDB/voltdb/tree/master/tests/scripts/plannertester

This could be a student project.