Consider only output to LOG instead of hardcoded stdout/stderr

[PragmaTwice]

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• [X] I had searched in the issues and found no similar issues.

Motivation

Refer to https://github.com/apache/incubator-kvrocks/pull/660#issuecomment-1165637383.

It is good to keep kvrocks with only one output location. But currently, kvrocks also outputs some information to stdout when logdir is set to a regular file, which can be confusing for users.

Solution

No response

Are you willing to submit a PR?

• [ ] I'm willing to submit a PR!