Refactoring: check function return Status marked as [[nodiscard]]

[torwig]

Check all the return values of the Status type if they are marked as [[nodiscard]].
Some functions were refactored so that now they return Status instead of bool or void; some functions now didn't return Status.

Closes: #1196

[PragmaTwice]

Great changes. Since all warnings are fixed, could you modify the CMakeFiles to make the warning as error? You can just follow this TODO, and put -Werror=unused-result to L181.

https://github.com/apache/incubator-kvrocks/blob/09d3d18dd9d4e2afebdda13560a8de392b0540d1/CMakeLists.txt#L180-L181

[torwig]

@PragmaTwice Done.

[torwig]

Thanks all. Merging...