add benchmark against sync.WaitGroup
add benchmark against sync.WaitGroup

My results: about 1.54x performance penalty

MrStrong/Sia,mnsl/Sia,mharkus/Sia,ericflo/Sia,ChrisSchinnerl/Sia,NebulousLabs/Sia,ChrisSchinnerl/Sia,MrStrong/Sia,ericflo/Sia,mharkus/Sia,mtlynch/Sia,rivine/rivine,rivine/rivine,mtlynch/Sia,mnsl/Sia,NebulousLabs/Sia