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".
Should we explain here why we are creating this event trigger? I think we should mention in these comments that we need to create directories and files after "CREATE SERVER" or "CREATE FOREIGN TABLE".