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
Preparing for release 4.5.2.Final: update Readme and changelog
Preparing for release 4.5.2.Final: apply workaround for release plugin not able to inject the Infinispan version
[maven-release-plugin] prepare release 4.5.2.Final
[maven-release-plugin] prepare for next development iteration
Revert "Preparing for release 4.5.2.Final: apply workaround for release plugin not able to inject the Infinispan version"

This reverts commit 5d0a104de63cee609459254c84201272c062e972 as it's not longer needed (nor desired) after the release.

Conflicts:
	modules/pom.xml