Cleanup UploadArtifactResourceImpl a little to determine if it is
causing the deadlock

Direct calls to loadArtifact don't seem to ever cause deadlocks. Infact,
they seem to be quite well synchronised, considering that
OWLOntologyManagerImpl is known not to be threadsafe
create new context-less statement for output
cleanup upload artifact resource some more
remove irrelevant javadoc
randomise file name used for temporary file to allow multiple files for
the same test
Add test for concurrency to upload artifact resource impl test that
reliably fails for me
downgrade to 8 as 9 seems to be triggering some strange restlet
behaviour