HSEARCH-1735 Clearly distinguish the Async from the BatchSync Processing...

... strategies and make sure no unnecessary thread is started

@gustavonalle one more change proposal for your new backend.

Best read the issue first:
https://hibernate.atlassian.net/browse/HSEARCH-1735
Ok I'll change that. I was not happy with the name.
The reason I included "batch" is that it works on List<LuceneWork>, which is essentially a block/batch of changes which should be applied atomically.