ChefSpec::Cacher is now thread safe.

Unfortunately could not test that FINALIZER will delete Thread cache. Tried this, but didn't work.

```ruby
it 'deletes thread data when thread finished' do
  Thread.new do
    runner = double(:runner)
    klass.cached(:chef_run){ runner }
    klass.new.chef_run
    expect(cache).not_to be_empty
  end.join
  expect(cache).to be_empty
end
```