Fix edge cases in Greenplum tracking view

1. Make sure a dropped table doesn't show up in the tracking view,
even if it was dropped outside the application via SQL in psql.
2. Remove ANALYZE and VACUUM ops from the tracking view since these
don't say anything about the updated-ness of the data in the tables
and they shouldn't override operations that do (e.g. insert, update).
3. pg_stat_operations has a null value for subtype for TRUNCATE
operations, so add a manual fix for this column for the TRUNCATE op.