Add support for ActiveRecord::Relation#load_async method.

[maxshend]

Hello, @DmitryTsepelev

A few days ago I tested how the gem works with ActiveRecord::Relation#load_async and as expected it didn't work because this method performs database queries outside of the main thread. But instantiation of models is done in the main thread. So I've created a patch for ActiveRecord::FutureResult class (it's used to get results from such queries).

I had to change DatabaseCleaner configurations because load_async inside transactions executes queries in the main thread.

Would like to know your opinion about this changes. Thank you!

[DmitryTsepelev]

LGTM, thank you! Let's try it out 🙂