modify tfs_upstream directives

1 delete some dead code.
2 fix some code style.
3 modify tfs_upstream directives and document.

tfs_upstream example:

    tfs_upstream tfs {
        server 127.0.0.1:6100;
        type rc_server;
        zone size=128M;
        net_device eth0;
        poll_rcs lock_file=/logs/lk.file interval=10s enable=1;
    }

    location / {
		tfs_pass tfs://tfs;
    }