seastar::sstring.find() is not compatible with std::string.find()

[longqimin]

    seastar::sstring empty = "";
    std::string std_empty = "";
    CHECK_EQ(empty.find(empty), std_empty.find(std_empty));

the above check will fail because seastar::sstring.find() return 4294967295 while std::string.find() returns 0.

[nyh]

This is not the only difference between seastar::sstring and std::string - see issue #634 for more, but you're right that this sort of difference is completely unnecessary and can and should be fixed. A patch (and an accompanying test) would be welcome :-)