Speedup librispeech dataset processing by using multiple processes.
PTAL. I addressed the ordering issue. LGTM will merge after tests pass
It is 2021, we should use parallel computations for slow scripts when possible.

As each directory contains independent data, we can easily add more processes to get_librispeech_data script to decrease waiting time.

On my machine the speedup when using 4 processes was 7-8 fold.
The whole download process went from >2h waiting time to 15 minutes.

The downloaded data is identical, so there should not be any problems introduced by parallelizing the script.