Set threads for augur subsample

[victorlin]

Take advantage of built-in parallelism.

Preview: https://nextstrain--267.org.readthedocs.build/en/267/guides/bioinformatics/filtering-and-subsampling.html#within-snakemake-workflow-config

Checklist

• Merge+release nextstrain/augur#1963, drop b30e0bd
• Checks pass
• Update changelog

[victorlin]

This is no longer applicable with proximal samples that can take advantage of multiple threads.

We should still update docs to mention augur subsample --nthreads, but the optimal number of threads depends on data, config, and how augur subsample fits into the rest of the workflow.