perf(core): Use chain.from_iterable in threading.py (python-zk#614)

This is a faster and more idiomatic way of using itertools.chain.
Instead of computing all the items in the iterable and storing them
in memory, they are computed one-by-one and never stored as a huge
list. This can save on both runtime and memory space.

kazoo/handlers/threading.py

@@ -178,7 +178,7 @@ def select(self, *args, **kwargs):
         # anything to minimize changes
         if _HAS_EPOLL:
             # if the highest fd we've seen is > 1023
-            if max(map(_to_fileno, chain(*args[:3]))) > 1023:
+            if max(map(_to_fileno, chain.from_iterable(args[:3]))) > 1023:
                 return self._epoll_select(*args, **kwargs)
         return self._select(*args, **kwargs)