Fix ConcurrentModificationException on commitLater (#476)

Invocations of consumer.commitAsync should be protected under the consumer semaphore, or you get errors like this when trying to call commitLater concurrently with poll:

java.util.ConcurrentModificationException: KafkaConsumer is not safe for multi-threaded access

Author: migesok

skafka/src/main/scala/com/evolutiongaming/skafka/consumer/Consumer.scala

@@ -310,7 +310,7 @@ object Consumer {
                 }
               }
             }
-            Sync[F].blocking(f(offsetCommitCallback)).as(Applicative[F].unit.some)
+            serialNonBlocking(f(offsetCommitCallback)).as(Applicative[F].unit.some)
           }
       }