Move close-and-cleanup functions to a postloop event.

Implements ticket 25932.

changes/ticket25932

@@ -0,0 +1,9 @@
+  o Minor features (mainloop):
+    - Move responsibility for
+      closing connections, circuits, and channels
+      from a once-per-second callback to a callback that is only scheduled as
+      needed.  Once enough items are removed from our once-per-second
+      callback, we can eliminate it entirely to conserve CPU when idle.
+      Closes ticket
+      25932.
+