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
I actually tried doing that but it was getting very hairy.
We need to handle the "live reboot" functionality because of Infinispan Query, and for that reason there is an LuceneBackendResources#onTheFlyRebuild() method in the resources.
Really the "resources" should collect all resources, I was actually tempted to move the Thread handling from the SyncProcessor to the "lucene resources" block so that it would be managed consistently with the async executor, but the thread would need to breack encapsulation anyway as it needs to be parked, etc..  so that was getting hairy and I backed out to keep this changeset simpler.
There is for sure some more reorganization to do.