o.c.opibuilder: Fix race condition in ResourceUtil URL Loader

First update to #370 made openURLStream () re-entrant, but left a race condition in place:
Both the original and the updated code checked the cache, then later read from cache... but the cache entry could expire in between those calls. 

Also using general Eclipse 'Job' instead of JFace & PlatformUI calls.
Unclear why Job/Thread in the first place, but leaving that unchanged because no time to check all call paths.