HSEARCH-1390 Avoid force unlock operations on unlocked directories
HSEARCH-1389 Avoid context switching on single-task indexing transactions
HSEARCH-1389 Some clean-up (removing superfluous method parameters, simplifying collection of erronous items)
HSEARCH-1391 ErrorContextBuilder should build error placeholders lazily
HSEARCH-1389 Avoiding list access for successfully processed work items

Conflicts:
	engine/src/main/java/org/hibernate/search/backend/impl/lucene/LuceneBackendQueueTask.java
HSEARCH-1391 Avoiding unchecked warning