pubsys: limit threads during validate-repo

[webern]

Issue number:

Closes #1522

Description of changes:

In a previous quick fix we spawned a thread for every target during
pubsys validate-repo. Now we limit the number of threads with a rayon
thread pool.

Testing done:

• I ran cargo make validate-repo for a production repo with no issues.
• I did the same for a personal dev repo, no issues.
• I manipulated a target in the dev repo and tried again and saw an appropriate failure:

Hash mismatch for [snip]/targets/[snip].migrate_v1.1.0_schnauzer-paws.lz4: calculated 2bad2467bcdf174dbb915978bdbcd18c0e79b457b8f52b92750ac5089b6c082e, expected ddc5fa22e8359c66789ce47f437b42652aab80d9a837be61e55cc42a46baca94

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

🐕

[webern]

webern force-pushed the webern:no-thread-bombing branch from 16cf3fc to c9f6fb7

Uh-oh that's a git mistake horror show. Reverting

[webern]

webern force-pushed the webern:no-thread-bombing branch from c9f6fb7 to 16cf3fc now

Puts us back to the initial sha the PR was opened with.

[webern]

webern force-pushed the webern:no-thread-bombing branch from 16cf3fc to 6dee545

implement @tjkirch's suggestions, this time without a git bomb.