(third_party deps) fix #177, update bundled spdlog to 1.6.0

third_party/spdlog/LICENSE

@@ -20,3 +20,7 @@ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 
+-- NOTE: Third party dependecy used by this sofware --
+This software depends on the fmt lib (MIT License),
+and users must comply to its license: https://github.com/fmtlib/fmt/blob/master/LICENSE.rst
+