Move consdiffmgr_rescan() into a mainloop event.

The change here was very simple, since there is a flag set whenever
we want to schedule this event.

Closes ticket 25391.
m

changes/ticket25931

@@ -0,0 +1,9 @@
+  o Minor features (mainloop):
+    - Move responsibility for
+      rescanning the consensus cache
+      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:
+      25931.
+