Use explicit namespaces for Mutex and ConditionVariable

[deivid-rodriguez]

This upstreams this PR to the rubygems repository: rubygems/rubygems#4709.

Using the explicit Thread namespace is the future-proof way of accessing these constants.

[deivid-rodriguez]

Thanks for merging! ❤️

I noticed though that according to rubygems/rubygems#4709 (comment), Thread::Mutex was introduced on ruby 2.3, and this library supports down to ruby 2.2 included. So this will break ruby 2.2 support.

I can:

• Create a PR bumping the minimum supported ruby to 2.3.
• Create a PR reverting this PR, and will keep our custom patch until you drop support.

Let me know which one you prefer.

[mperham]

We can drop 2.2, but let’s bump our version to 2.3 and add it to the changelog.