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".
Minor. In similar cases we usually do "else if" instead of "if". If we use "else if", we should also remove the empty line before the comment.