HSEARCH-1725 Improve allocation rate of the new Lucene synchronous batching backend

Conflicts:
	engine/src/main/java/org/hibernate/search/backend/impl/lucene/BatchSyncProcessor.java
	engine/src/main/java/org/hibernate/search/backend/impl/lucene/ChangesetList.java
HSEARCH-1735 Clearly distinguish the Async from the BatchSync Processing strategies and make sure no unnecessary thread is started

Conflicts:
	engine/src/main/java/org/hibernate/search/backend/impl/lucene/LuceneBackendResources.java