Add queue to workers randomly

Round robin is not thread safe.

It would require putting an atomic on ``next_worker``
which would probably slow dispatching queues.

Hence prefering random dispath.

kdtree/runtime/runtime.pxd

@@ -104,7 +104,6 @@ cdef cypclass Scheduler:
     pthread_barrier_t barrier
     sem_t num_free_queues
     atomic[int] num_pending_queues
-    int next_worker
     int num_workers
     sem_t done
     volatile bint is_done
@@ -113,7 +112,6 @@ cdef cypclass Scheduler:
         self = <lock Scheduler> consume alloc()
         if num_workers == 0: num_workers = sysconf(_SC_NPROCESSORS_ONLN)
         self.num_workers = num_workers
-        self.next_worker = 0
         sem_init(&self.num_free_queues, 0, 0)
         sem_init(&self.done, 0, 0)
         self.num_pending_queues.store(0)
@@ -141,9 +139,8 @@ cdef cypclass Scheduler:
         sem_destroy(&self.done)
 
     void post_queue(self, lock SequentialMailBox queue):
-        # Add a queue to workers in round robin
-        worker = self.workers[<int> self.next_worker]
-        self.next_worker = (self.next_worker + 1) % self.num_workers
+        # Add a queue to workers randomly
+        worker = self.workers[<int> rand() % self.num_workers]
         with wlocked worker:
           queue.has_worker = True
           worker.queues.push_back(queue)