Multithreaded retrieval of samples in storage to supervisor

[MaxiBoether]

The supervisor expects all new samples in order. The current (disabled) multithreaded version of send_file_ids_and_labels however sends the new samples in arbitrary order since the threads just send data when available. Hence, we needed to disable the multithreaded implementation for now and enforce single threaded sending of new samples.

We might want to parallelize building the result and then sending it in order or something else that speeds this up.