Summary:
`Slice::Less` is inconsistent with `Slice::compare` which is reproduced by the following test:
```
str1 = "ABC";
str2 = "";
const auto s1 = Slice(str1);
const auto s2 = Slice(str2);
ASSERT_EQ(s1.Less(s2), s1.compare(s2) < 0);
```
Fixed `Slice::Less` and added SliceTest.OrderConsistency random test.
Jira: DB-14523
Test Plan: ybd --gtest_filter SliceTest.OrderConsistency -n 50 -- -p 1 for release/debug/asan/tsan builds
Reviewers: arybochkin, sergei
Reviewed By: arybochkin, sergei
Subscribers: sergei, ybase
Tags: #jenkins-ready
Differential Revision: https://phorge.dev.yugabyte.com/D40717