Support rails parallel testing

[quorak]

Make index_name thread save to support parallel-test feature from rails

It looks like setting the index_name per test can lead to cross pollination of the index_name, leading to situation where
the index changes during test, because another elasticsearch test just started.

• Are we sure, current implementation even can (and want) support highly concurrent test scenarios with different indexes?
• Can you thin of another way of speedin gup tests?

I checked #342 but don't think naming will solve everything