ThreadService: track parent threads globally

At a given time, each Thread can only have one parent Thread, even
if there are multiple application Contexts in play each with its
own ThreadService.

Without this change, the new getThreadContext method will sometimes fail
to return ThreadContext.OTHER as expected, because thread parents were
only tracked by the spawning ThreadService, and hence unavailable to the
other ThreadService doing the interrogation.

Author: ctrueden

src/main/java/org/scijava/thread/DefaultThreadService.java

@@ -57,6 +57,9 @@ public final class DefaultThreadService extends AbstractService implements
 
 	private static final String SCIJAVA_THREAD_PREFIX = "SciJava-";
 
+	private static WeakHashMap<Thread, Thread> parents =
+		new WeakHashMap<Thread, Thread>();
+
 	@Parameter
 	private LogService log;
 
@@ -66,8 +69,6 @@ public final class DefaultThreadService extends AbstractService implements
 
 	private boolean disposed;
 
-	private WeakHashMap<Thread, Thread> parents = new WeakHashMap<Thread, Thread>();
-
 	// -- ThreadService methods --
 
 	@Override