Disable WAL and flushing for RocksDB writes

[tillrohrmann]

With #1216 we are relying on the log for storing the ground truth. Therefore, it is no longer needed that the PartitionProcessor writes durably to its state store. Concretely, I believe it should be possible to disable writing to the WAL of RocksDB and syncing the data to disk. The important bit is that atomic flushes are used as long as we are reusing the stored data from a previous run.