Fix a race condition that may occur when using InfModel in a multi-threaded environment, it was previously possible for multiple threads to try and call prepare() and hit ConcurrentModificationException's.

This fix modifies the code for the underlying InfGraph() to make calls to prepare() synchronized and to ensure threads always see the current prepared state.  This should resolve JENA-399, the previously failing tests are now enabled

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1448883 13f79535-47bb-0310-9956-ffa450edef68