libev: service timeouts before updating loop timer.

Also, never start a timer with zero repeat (this causes it to stop,
rather than timeout immediately).

cassandra/io/libevreactor.py

@@ -148,6 +148,7 @@ class LibevLoop(object):
 
     def _update_timer(self):
         if not self._shutdown:
+            self._timers.service_timeouts()
             offset = self._timers.next_offset or 100000  # none pending; will be updated again when something new happens
             self._loop_timer.start(offset)
         else:

cassandra/io/libevwrapper.c

@@ -517,7 +517,9 @@ Timer_start(libevwrapper_Timer *self, PyObject *args) {
     if (!PyArg_ParseTuple(args, "d", &timeout)) {
         return NULL;
     }
-    self->timer.repeat = fmax(timeout, 0.0);
+    /* some tiny non-zero number to avoid zero, and
+     make it run immediately for negative timeouts */
+    self->timer.repeat = fmax(timeout, 0.000000001);
     ev_timer_again(self->loop->loop, &self->timer);
     Py_RETURN_NONE;
 }