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 use remove() or unlink()? I am not sure, but unlink() is used much more in the PostgreSQL source code and also it is defined in port.h, so it might be a more portable option.