Refactor StreamedRDFIterator to use blocking polling in favour of incremental back off retry with non-blocking polling.  Improve tests to use semaphore to avoid thread contention issues leading to failing test runs

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1439498 13f79535-47bb-0310-9956-ffa450edef68