Fix Thread leaking in ThreadLocalVariables

- introduce #bind method to avoid value leaks
- warn on using just #value=
It's a very good point the 4th one. I agree we should either reimplement or pull the gem under concurrent-ruby in long-term. Technically it passes 3 because on JRuby the gem is not being loaded so `gem 'ref', platform: :mri` will avoid any problems with compilation on JRuby, that should make the short-term solution even more viable.