TABLE::use_all_columns turn on all bits of read_set, which is
interpreted by innodb as a request to read all columns. Without doing
so before calling init_read_record(), innodb will not retrieve any
columns if mysql.servers table has been altered to use innodb as the
engine, and any foreign servers stored in the table are "lost".
The limit of socket length on unix according to libc is 108, see
sockaddr_un::sun_path, but in the table it is a string of max length
64, which results in truncation of socket and failure to connect by
plugins using servers such as spider.