fix(core): when connection fails, close the ConnectionHandler (#577) (#579)

When connection attempts fail repeatedly (e.g. all ZK servers are
unavailable), eventually the socketpair in the ConnectionHandler fills
up, and the Client gets stuck trying to write a single byte to the
socketpair.

Avoid this by ensuring we close the socketpair on a failed connection
attempt.

Author: saffroy

kazoo/client.py

@@ -628,6 +628,7 @@ def start(self, timeout=15):
         if not self.connected:
             # We time-out, ensure we are disconnected
             self.stop()
+            self.close()
             raise self.handler.timeout_exception("Connection time-out")
 
         if self.chroot and not self.exists("/"):