trivial: replace x=x+y with x+=y
trivial: replace x=x+y with x+=y

Also organize imports according to the go-import convention used everywhere else
already.

HelixEducation/sarama,scalingdata/sarama,raisemarketplace/sarama,VividCortex/sarama,kraveio/sarama,jepst/sarama,Shopify/sarama,kmiku7/sarama,crast/sarama,remerge/sarama,LastManStanding/sarama,cep21/sarama,yangfan876/sarama,jimmykuo/sarama,cep21/sarama,mailgun/sarama,vrischmann/sarama,kmiku7/sarama,LastManStanding/sarama,VividCortex/sarama,remerge/sarama,DataDog/sarama,yangfan876/sarama,aybabtme/sarama,pronix/sarama,rafrombrc/sarama,rafrombrc/sarama,jepst/sarama,crast/sarama,mailgun/sarama,HelixEducation/sarama,kraveio/sarama,DataDog/sarama,jimmykuo/sarama,uovobw/sarama,vrischmann/sarama,winlinvip/sarama,Neuw84/sarama,aybabtme/sarama,pronix/sarama,scalingdata/sarama,raisemarketplace/sarama,winlinvip/sarama,Shopify/sarama,Neuw84/sarama,uovobw/sarama