Make the yaml cache fully thread-safe

[mensfeld]

Concurrent::Map.new { |h, k| h[k] = Concurrent::Map.new }

is prone to a race condition, thus the cache won't be fully thread safe if definitions are parsed in multiple threads.

This PR fixes that.

Ref: ruby-concurrency/concurrent-ruby#970

[mensfeld]

specs dont seem to be related to this change

[solnic]

I'm afraid spec failures are indeed caused by the fix, seems like cache isn't cleared properly between spec runs now.

[mensfeld]

Got it. WIll look at this today.

[flash-gordon]

I have main failing locally too, investigating

Finished in 10.7 seconds (files took 3.15 seconds to load)
3183 examples, 39 failures, 4 pending

[flash-gordon]

main is green, merging

[granthusbands]

@mensfeld You missed the ||= that you've fixed in pull requests for other projects.

[mensfeld]

@granthusbands I did not miss it. I explicitly do not do it here to make the change small. I pointed out in the article that I wanted to keep the scope small because I do not know how given libs operate.

that you've fixed in pull requests for other projects.

Not exactly the same. In most cases, I fixed the internal ||=, not the external. Nonetheless you are right and feel free to create a PR :) @solnic should be happy