Use explicit namespaces for `Mutex` and `ConditionVariable` (#153)
Use explicit namespaces for `Mutex` and `ConditionVariable` (#153)

This upstreams this PR to the rubygems repository:
https://github.com/rubygems/rubygems/pull/4709.

Using the explicit `Thread` namespace is the future-proof way of
accessing these constants.
mperham/connection_pool,ttasanen/connection_pool