this the standard library offers
`std::lexicographical_compare_threeway()`, and we never uses the
last two addition parameters which are not provided by
`std::lexicographical_compare_threeway()`. there is no need to have
the homebrew version of trichotomic compare function.
in this change,
* all occurrences of `lexicographical_tri_compare()` are replaced
with `std::lexicographical_compare_threeway()`.
* ``lexicographical_tri_compare()` is dropped.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#13615