SLING-2535 Undid my previous commit and put the synchronisation one layer further out as orriginally suggested. Synchronising in incUsage and decUsage is too risky from a deadlock point of view. Added Javadoc to warn about thread safety on the methods.
sling/trunk/bundles/commons/threads/src/main/java/org/apache/sling/commons/threads/impl/DefaultThreadPoolManager.java