This is one of our esoteric metadata mappers so I think we should distribute
it in a plugin rather than in elasticsearch core.
This introduces one limitation: the value of the `_size` parameter is not
retrievable for documents that are only in the transaction log.
We have a smoke_test_plugins.py, but its a bit slow, not integrated
into our build, etc.
I converted this into an integration test. It is definitely uglier
but more robust and fast (e.g. 20 seconds time to verify).
Also there is refactoring of existing integ tests logic, like printing
out commands we execute and stuff