Candidate fix for issue #55 NPE in LocationInfo at end of test.

Appears that a null LocationInfo is making it into the ArrayList of LocationInfos, despite a null check guard.
Expect this is caused by a multi-threaded race between client worker and server worker during test teardown.
Updated relevant data structures to their concurrent equivalents.
Collection.sort relies on Iterator.set(...) which is not supported by CopyOnWriteArrayList.
Merge pull request #56 from jfallows/develop

Candidate fix for issue #55 NPE in LocationInfo at end of test.