internal flush function for logger
internal flush function for logger

lisong521/easyloggingpp,spqr33/easyloggingpp,spthaolt/easyloggingpp,spqr33/easyloggingpp,simonhang/easyloggingpp,spthaolt/easyloggingpp,blankme/easyloggingpp,arvidsson/easyloggingpp,dreal-deps/easyloggingpp,hellowshinobu/easyloggingpp,utiasASRL/easyloggingpp,simonhang/easyloggingpp,arvidsson/easyloggingpp,simonhang/easyloggingpp,chenmusun/easyloggingpp,lisong521/easyloggingpp,arvidsson/easyloggingpp,blankme/easyloggingpp,spqr33/easyloggingpp,blankme/easyloggingpp,hellowshinobu/easyloggingpp,spthaolt/easyloggingpp,dreal-deps/easyloggingpp,utiasASRL/easyloggingpp,chenmusun/easyloggingpp,chenmusun/easyloggingpp,utiasASRL/easyloggingpp,hellowshinobu/easyloggingpp,dreal-deps/easyloggingpp,lisong521/easyloggingpp