Added functionality to delete files automatically on DROP FOREIGN TABLE

This change adds functionality to allow cstore_fdw to delete files automatically on DROP FOREIGN TABLE command. It also fixes issues "cstore_fdw should error out on create if it cannot access the file" and "cstore_fdw should return empty table if no data has been loaded".
Do this line and the previous line work properly if !CALLED_AS_EVENT_TRIGGER()? If not, we can move the assignments to after the if (!CALLED_AS_EVENT_TRIGGER()) check.