OAK-1666 - FileDataStore inUse map causes contention in concurrent env

Temporary workaround which uses a NoOpMap for inUseMap untill fix for JCR-3764 is ready

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1583994 13f79535-47bb-0310-9956-ffa450edef68