Switched from single-node (assign) to multi-instance (subscribe)

* TransferConsumer uses subscribe() instead of assign().
* The subscription happens during the instanciation of TransferConsumer.
* The restorage-process of the state happens in onPartitionsAssigned().
* To be able to reset all data, that belongs to a specific partition
  after a rebalance, in order to avoid state errors because of handling
  events several times, InMemoryTransferRepository must know, which
  partition a transfer belongs to.
* To achieve this, the partitioning algorithm is made known explicitly.
* On each rebalance, a mapping from the partions to the currently assigned
  instances is maintained.
* TransferController uses the explicitly known partitioning algorithm, to
  look up the partion for a requested transfer and decide, if the data is
  available locally.
* If not, it looks up the assigned instance in the maintained mapping and
  redirects the request.