KafkaMLC - make addOffsets() thread-safe

[garyrussell]

https://stackoverflow.com/questions/47652651/spring-kafka-is-acknowledgement-acknowledge-thread-safe/47663459#comment84501434_47663459

[lhsm]

Hi, seems still not thread safe. Please, take a look into KafkaMessageListenerContainer.ListenerConsumer#buildCommits. While iterating offsets new offset may be added
The same in KafkaMessageListenerContainer.ListenerConsumer#commitPendingAcks

[garyrussell]

Good catch; thanks.