HSEARCH-1716 Simplify the Lucene writing backend by dropping the paralle...

...l execution of writer tasks

Some changes in the test were needed, I believe the new outcome (the assertions of the output) are much better too as it is much closer to the user expectation.
Previously it was harder to understand what happened as the state had to be de-parallelized on error, and the error messages was puzzling (although correct). Now it's simpler.

 - https://hibernate.atlassian.net/browse/HSEARCH-1716