stats: Another attempt at fixing the race condition in flushing.

It appears that sometimes multiple threads are flushing the same dict at
the same time. Yikes! This should fix that without double counting.

r2/r2/lib/stats.py

@@ -53,8 +53,12 @@ class TimingStatBuffer:
     def flush(self):
         """Yields accumulated timing and counter data and resets the buffer."""
         data, self.data = self.data, collections.defaultdict(complex)
-        while data:
-            k, v = data.popitem()
+        while True:
+            try:
+                k, v = data.popitem()
+            except KeyError:
+                break
+
             total_time, count = v.real, v.imag
             yield k, str(int(count)) + '|c'
             divisor = count or 1