Updated the benchmark for better spdlog results

Hi,
I added the following for better spdlog results:

1. spdlog async works best when max queue size is not so big.
2. Added spdlog sync test (which is faster than async  - as async mode is still work in progress)
   
Anyway, it is good stuff. Going to borrow ideas (and maybe code if ok :) for future spdlog async improvments..

Would be nice if you rerun and publish new results
Merge pull request #1 from gabime/master

Updated the benchmark for better spdlog results