TransferRepository does not need any synchronization

* Only TransferService stores data in the repository
* Since alle instances of Transfer, that are handled by TransferService
  are received through a single topic, no synchronization is needed at
  all in the repository.
* This is, because records, that are received from a topic are guaranteed
  to be processed one after the other.
* The topic simply is the single source for processing requests and a
  KafkaConsumer never handles multiple records in parallel.
* Note: This implementation is not ready to run on multiple threads or
  nodes!