remove race where event loop deletes watcher during cleanup

PYTHON-578

cassandra/io/libevreactor.py

@@ -121,10 +121,7 @@ class LibevLoop(object):
 
         for conn in self._live_conns | self._new_conns | self._closed_conns:
             conn.close()
-            if conn._write_watcher:
-                conn._write_watcher.stop()
-            if conn._read_watcher:
-                conn._read_watcher.stop()
+            map(lambda w: w.stop(), (w for w in (conn._write_watcher, conn._read_watcher) if w))
 
         self.notify()  # wake the timer watcher
         log.debug("Waiting for event loop thread to join...")