ZookeeperClient.close() should cancel scheduled_timeout

[Robert Thille]

If a caller of ZookeeperClient attempts a connection, and then, before the connection completes calls ZookeeperClient.close(), the 'scheduled_timeout' (reactor.callLater() call back) is not cancelled and will cause the client to get a spurious errback() on the connect deferred.

the scheduled_timeout created here:
http://bazaar.launchpad.net/~juju/txzookeeper/trunk/view/head:/txzookeeper/client.py#L465
should be stored away so that it's accessible from close(), so it can be cancelled.