Garbage collect deactivated timers when they are added faster than they can be triggered.

src/timer.d

@@ -147,6 +147,19 @@ public:
 
     final size_t addCounter(uint length, bool delegate() expiry)
     {
+        if (nextFree == counters.length)
+        {
+            size_t idx = head;
+            size_t prev = -1;
+            while (idx != -1)
+            {
+                if (!counters[idx].active)
+                    deleteCounter(idx, prev);
+                else
+                    prev = idx;
+                idx = counters[idx].next;
+            }
+        }
         if (nextFree == counters.length)
         {
             counters.length += 20;