minor improvements on flush for logger
minor improvements on flush for logger

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