don't perform costly 'difference_update' on dependencies

Scheduler:

set1.difference_update(set2) can be very slow if set2 is large,
even if set1 is empty, so use faster set1 = set1.difference(set2), and 
don't perform any difference operation on empty dependencies.

The result was a dramatic reduction in throughput after the scheduler has 
routed many thousands of tasks, even with no task dependencies.

IPython/parallel/controller/scheduler.py

@@ -223,7 +223,6 b' class TaskScheduler(SessionFactory):'
             except KeyError:
                 self.log.error("task::Invalid notification msg: %r"%msg)
     
-    @logged
     def _register_engine(self, uid):
         """New engine with ident `uid` became available."""
         # head of the line:
@@ -322,16 +321,20 b' class TaskScheduler(SessionFactory):'
         self.retries[msg_id] = retries
         
         # time dependencies
-        after = Dependency(header.get('after', []))
+        after = header.get('after', None)
+        if after:
+            after = Dependency(after)
         if after.all:
             if after.success:
-                after.difference_update(self.all_completed)
+                    after = after.difference(self.all_completed)
             if after.failure:
-                after.difference_update(self.all_failed)
+                    after = after.difference(self.all_failed)
         if after.check(self.all_completed, self.all_failed):
             # recast as empty set, if `after` already met,
             # to prevent unnecessary set comparisons
             after = MET
+        else:
+            after = MET
         
         # location dependencies
         follow = Dependency(header.get('follow', []))
@@ -345,6 +348,8 b' class TaskScheduler(SessionFactory):'
         
         # validate and reduce dependencies:
         for dep in after,follow:
+            if not dep: # empty dependency
+                continue
             # check valid:
             if msg_id in dep or dep.difference(self.all_ids):
                 self.depending[msg_id] = args