Revert automatic cleanups made in JobManager

Revert unintentional changes made in
5919685

See eclipse-platform#1282 (comment)

Contributes to
eclipse-platform#664

runtime/bundles/org.eclipse.core.jobs/src/org/eclipse/core/internal/jobs/JobManager.java

@@ -907,8 +907,10 @@ protected boolean isBlocking(InternalJob runningJob) {
 			while (previous != null) {
 				// ignore jobs of lower priority (higher priority value means lower priority)
 				if (previous.getPriority() < runningJob.getPriority()) {
+					if (!previous.isSystem())
+						return true;
 					// implicit jobs should interrupt unless they act on behalf of system jobs
-					if (!previous.isSystem() || (previous instanceof ThreadJob && ((ThreadJob) previous).shouldInterrupt()))
+					if (previous instanceof ThreadJob && ((ThreadJob) previous).shouldInterrupt())
 						return true;
 				}
 				previous = previous.previous();
@@ -953,8 +955,10 @@ protected boolean join(InternalJob job, long timeout, IProgressMonitor monitor)
 		final Semaphore barrier;
 		synchronized (lock) {
 			int state = job.getState();
+			if (state == Job.NONE)
+				return true;
 			//don't join a waiting or sleeping job when suspended (deadlock risk)
-			if ((state == Job.NONE) || (suspended && state != Job.RUNNING))
+			if (suspended && state != Job.RUNNING)
 				return true;
 			//it's an error for a job to join itself
 			if (state == Job.RUNNING && job.getThread() == Thread.currentThread())
@@ -1053,8 +1057,10 @@ public void scheduled(IJobChangeEvent event) {
 						Job job = event.getJob();
 						if (family == null || job.belongsTo(family)) {
 							// don't add to list if job is being rescheduled
+							if (((JobChangeEvent) event).reschedule)
+								return;
 							// if job manager is suspended we only wait for running jobs
-							if (((JobChangeEvent) event).reschedule || isSuspended())
+							if (isSuspended())
 								return;
 							boolean added = jobs.add(job);
 							assert added;