Replace `ActiveSupport::Concurrency::Latch` with `Concurrent::CountDownLatch` from concurrent-ruby.

[jdantonio]

This update was suggested by @tenderlove in a discussion on one of his PRs to concurrent-ruby.

The concurrent-ruby gem is a toolset containing many concurrency utilities. Many of these utilities include runtime-specific optimizations when possible. Rather than clutter the Rails codebase with concurrency utilities separate from the core task, such tools can be superseded by similar tools in the more specialized gem. This commit replaces ActiveSupport::Concurrency::Latch with Concurrent::CountDownLatch, which is functionally equivalent.

[tenderlove]

@jdantonio thanks. I don't think this is enough though. We have runtime requirements for the countdown latch. Can you update the gemspecs that need concurrent-ruby to depend on it? I think only actionpack needs it.

Also we may want to consider adding a deprecation warning for the countdown latch that's in Rails (though since this is Rails 5, and I don't think many people use our latch I'm not sure we need that).

[jdantonio]

Sorry I missed the runtime requirement. I thought it was only used in tests. I'll update ASAP.

[tenderlove]

@jdantonio thanks so much! Also, if you don't mind, could you check our uses of the thread_safe gem? AFAICT, concurrent-ruby supersedes it, and I'd rather just switch thread_safe to concurrent-ruby if that's possible.

[jdantonio]

@tenderlove We have not merged thread_safe into concurrent-ruby yet, just ruby-atomic. We want to get c-r 1.0 out before we merge thread_safe.

[tenderlove]

I see. Nevermind about thread_safe then. :)

[jdantonio]

@tenderlove I put ActiveSupport::Concurrency::Latch back but it now extends Concurrent::CountDownLatch and gives a deprecation warning.

I removed c-r from Gemfile and put the c-r dependency in the gemspec for activesupport (since it now uses CountDownLatch directly). Both actionpack and activerecord use CountDownLatch in their tests but both should pick up the dependency through activesupport.

I couldn't find any place in actionpack that used the original Latch and the tests all passed on the original commit. I added c-r to its gemspec anyway, at your suggestion.