Update single job scripts like utils/data/get_utt2dur.sh, steps/compute_cmvn_stats.sh to use multiple jobs

In the large data recipes in Kaldi many single job scripts in steps/\* and utils/*, which were very fast for normal sized LVCSR tasks, consume several hours.
e.g. on fisher_swbd/aspire the `utils/data/get_utt2dur.sh` script runs for a few hours.

We need to update these scripts to add parallelization support.

Required skill: shell scripting

Start by comparing the named scripts, utils/data/get_utt2dur.sh and steps/compute_cmvn_stats.sh, with the large data recipes described in the issue. Trace how each script processes its inputs and identify what must change to support multiple jobs. Done means the affected single-job scripts can use parallelization without breaking normal-sized LVCSR workflows.

Written by the indexing model from the issue text.