Stop using event_base_once().

This function leaks memory when the event_base is freed before the
event itself fires.  That's not harmful, but it's annoying when
trying to debug other memory leaks.

Fixes bug 24584; bugfix on 0.2.8.1-alpha.

changes/bug24584

@@ -0,0 +1,5 @@
+  o Minor bugfixes (memory cleanup):
+    - Avoid possible at-exit memory leaks related to use of Libevent's
+      event_base_once() function. (This function tends to leak memory
+      if the event_base is closed before the event fires.) Fixes bug 24584;
+      bugfix on 0.2.8.1-alpha.