Require only necessary concurrent-ruby classes.

[jdantonio]

This PR has two changes, which I will happily separate into two PRs if that would be preferred. The two changes are:

• Using concurrent-ruby v1.0.0.pre5 (upgraded from pre3)
• Limiting require statements to only load necessary files from concurrent-ruby

1.0.0.pre5

This is the last planned pre-release of concurrent-ruby. There are no new features, just performance improvements and a few refinements to the internal synchronization layer and associated memory model. We hope to release 1.0.0 fairly soon, once our biggest users have upgraded to pre5.

Updated require statements

Based on a recommendation by @jeremy in a concurrent-ruby issue.

For historical reasons, most of c-r doesn't use autoload (though we plan to change this in 2.0). By requiring the entire gem we unintentionally added unnecessary delay to Rails loading. Internally we have been very judicious in using explicit requires in all files. This makes it possible to include only the required c-r components. This update narrows all require statements to the minimum necessary. This should improve load time for all users.

[rails-bot]

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[jeremy]

🙌