Solve race condition in AsyncTargetWrapper
Solve race condition in AsyncTargetWrapper

Solves the race condition occuring when flushin the target twice in a row.
Solved by using a thread-safe queue to contain all the continuations to
call.
closes #214

RRUZ/NLog,babymechanic/NLog,AqlaSolutions/NLog-Unity3D,thomkinson/NLog,mikkelxn/NLog,ilya-g/NLog,Niklas-Peter/NLog,tetrodoxin/NLog,bhaeussermann/NLog,rajk987/NLog,zbrad/NLog,vbfox/NLog,UgurAldanmaz/NLog,FeodorFitsner/NLog,luigiberrettini/NLog,bhaeussermann/NLog,mikkelxn/NLog,thomkinson/NLog,AqlaSolutions/NLog-Unity3D,AqlaSolutions/NLog-Unity3D,MartinTherriault/NLog,czema/NLog,AndreGleichner/NLog,MoaidHathot/NLog,fringebits/NLog,rajk987/NLog,thomkinson/NLog,Niklas-Peter/NLog,kevindaub/NLog,RichiCoder1/NLog,vbfox/NLog,BrutalCode/NLog,breyed/NLog,pwelter34/NLog,campbeb/NLog,pwelter34/NLog,fringebits/NLog,zbrad/NLog,czema/NLog,FeodorFitsner/NLog,campbeb/NLog,bryjamus/NLog,littlesmilelove/NLog,hubo0831/NLog,thomkinson/NLog,ajayanandgit/NLog,ie-zero/NLog,tmusico/NLog,fringebits/NLog,ilya-g/NLog,bjornbouetsmith/NLog,ArsenShnurkov/NLog,breyed/NLog,czema/NLog,BrutalCode/NLog,ajayanandgit/NLog,rajk987/NLog,tohosnet/NLog,RRUZ/NLog,tmusico/NLog,nazim9214/NLog,michaeljbaird/NLog,luigiberrettini/NLog,matteobruni/NLog,vladikk/NLog,MoaidHathot/NLog,czema/NLog,bryjamus/NLog,304NotModified/NLog,vladikk/NLog,vladikk/NLog,MartinTherriault/NLog,BrandonLegault/NLog,NLog/NLog,AndreGleichner/NLog,matteobruni/NLog,sean-gilliam/NLog,hubo0831/NLog,babymechanic/NLog,snakefoot/NLog,tohosnet/NLog,littlesmilelove/NLog,mikkelxn/NLog,vladikk/NLog,kevindaub/NLog,mikkelxn/NLog,AqlaSolutions/NLog-Unity3D,ArsenShnurkov/NLog,rajeshgande/NLog,fringebits/NLog,nazim9214/NLog,bjornbouetsmith/NLog,RichiCoder1/NLog,BrandonLegault/NLog,tetrodoxin/NLog,UgurAldanmaz/NLog,rajk987/NLog,BrandonLegault/NLog,BrandonLegault/NLog,ie-zero/NLog,rajeshgande/NLog,michaeljbaird/NLog