crimson/os: rewrite ordering using std::strong_ordering

crimson/os: rewrite ordering using std::strong_ordering

the goals are

1. to use std::strong_ordering mechinary for better readability
   and maintainbility
2. to remove unnecessary abstraction
3. use concept for better error message and readability.

changes:

* replace MatchKindCMP with std::strong_ordering
* replace compare_to() with operator<=>
* introduce a concept `IsFullKey` so we can use it when developing
  generic facilities to operate on both materialized key or view.
* use `IsFullKey` in place of `KeyT` when appropriate

Signed-off-by: Kefu Chai <tchaikov@gmail.com>