Make ChefSpec::Cacher threadsafe

Unfortunately could not test that the `FINALIZER` will delete the Thread cache. I 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
```