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
Thinking of this class name, WDYT calling it ```WorkProcessor``` only? The 'batch' word is being used in several places already: ```BatchBackend```, ```BatchedQueueingProcessor```,```BatchCoordinator``` to name a few :smile: