receiving more than 6 events at the same time causing events lost

[teou]

https://github.com/samuel/go-zookeeper/blob/master/zk/conn.go#L297

receiving more than 6 events at the same time, the 7th event will be lost.

for example, receiving events in following order(processing events may be slow): StateConnected StateDisconnected StateDisconnected StateDisconnected StateDisconnected StateConnected StateHasSession

then StateHasSession will be lost.