client.pool: Optimise iterateForObject for most likely cases.

Nodes are likely to be running, so filtering before sort is unlikely to
save time. Caller is likely to stop iterating after first yield connection
("load" case), so move filtering inside loop.

Also, document non-straightforward code.

neo/client/pool.py

@@ -122,16 +122,21 @@ class ConnectionPool(object):
         getConnForNode = self.getConnForNode
         while cell_list:
             new_cell_list = []
-            cell_list = [c for c in cell_list if c.getNode().isRunning()]
+            # Shuffle to randomise node to access...
             shuffle(cell_list)
+            # ...and sort with non-unique keys, to prioritise ranges of
+            # randomised entries.
             cell_list.sort(key=self.getCellSortKey)
             for cell in cell_list:
                 node = cell.getNode()
-                conn = getConnForNode(node)
-                if conn is not None:
-                    yield (node, conn)
-                elif node.isRunning():
-                    new_cell_list.append(cell)
+                if node.isRunning():
+                    conn = getConnForNode(node)
+                    if conn is not None:
+                        yield (node, conn)
+                    # Re-check if node is running, as our knowledge of its
+                    # state can have changed during connection attempt.
+                    elif node.isRunning():
+                        new_cell_list.append(cell)
             cell_list = new_cell_list
             if new_cell_list:
                 # wait a bit to avoid a busy loop