Fix broker asyncness
Fix broker asyncness

You can actually have a second thread send while the first is still waiting for
a response now. The previous changes had the sender hold the lock the entire
time, which was silly. That's what the channel is for.

vrischmann/sarama,aybabtme/sarama,raisemarketplace/sarama,crast/sarama,pronix/sarama,HelixEducation/sarama,rafrombrc/sarama,DataDog/sarama,mailgun/sarama,winlinvip/sarama,jimmykuo/sarama,pronix/sarama,raisemarketplace/sarama,Shopify/sarama,remerge/sarama,scalingdata/sarama,VividCortex/sarama,kraveio/sarama,jimmykuo/sarama,VividCortex/sarama,uovobw/sarama,sclasen/sarama,crast/sarama,gyroscope-innovations/legacy_sarama,jepst/sarama,winlinvip/sarama,LastManStanding/sarama,cep21/sarama,yangfan876/sarama,remerge/sarama,Neuw84/sarama,kmiku7/sarama,kmiku7/sarama,LastManStanding/sarama,Neuw84/sarama,betable/sarama,gyroscope-innovations/legacy_sarama,mailgun/sarama,rafrombrc/sarama,aybabtme/sarama,HelixEducation/sarama,uovobw/sarama,betable/sarama,kraveio/sarama,DataDog/sarama,yangfan876/sarama,vrischmann/sarama,xiaotianlis/sarama,scalingdata/sarama,jepst/sarama,Shopify/sarama,cep21/sarama