feature: unify code comments

[hustjieke]

Is your feature request related to a problem? Please describe.

Currently the comment in StoneDB is not unified:

/*...*/
//
///
...

Describe the solution you'd like

For single line comment: //
For multi line comments: /* .... */

Describe alternatives you've considered

Additional context

[hustjieke]

Follow google style: https://google.github.io/styleguide/cppguide.html#Comment_Style

[RingsC]

This is a part of #11 , therefore, firstly, we close this, and reopen it if necessary.