Rework the TDB disk location locking a little and improve test casesto better detect whether they are valid on the platform they are running on (JENA-648)

Also adds a flag for this feature to SystemTDB which is currently off by default until the code and implementation stabilises

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1601601 13f79535-47bb-0310-9956-ffa450edef68