Successive calls to `start_async` deadlock

[gaohuazuo]

On kazoo 2.5 (current pip version), calling start_async twice in a row will cause the second call block indefinitely.

import kazoo.client
client = kazoo.client.KazooClient()
client.start_async(); client.start_async()

[StephenSorriaux]

Hi, thanks for your issue.

Kazoo 2.6.0 is available since yesterday but it does not address this issue.

My guess is that we should detect if the client is already connected or not. PR are welcomed if you have some times to fix this.