Improve benchmark: synchronous spdlog

[Theramar]

According to this issue it would be interesting to compare fmtlog with synchronous spdlog

[MengRao]

spdlog is already being compared with default synchrouous version, you can check the "bench" branch for details.

[hanzz2007]

fmtlog use async logging inside while spdlog here be configured sync logging. It is an unfair comparision.