Namenode hangs.

[pelmers]

Namenode does not respond to any inputs with final message
INFO [default] : **ZkWrapper** : [health child] Receive a heartbeat. A child has been added under path/health/127.0.0.1:50020
I cannot ctrl-c the process, nor will it respond to any client rpc.

On current develop branch, seems like a zookeeper issue, or some kind of memory corruption maybe.

Steps to reproduce:

1. Start namenode.
2. Start datanode.

The hang does not appear if I start datanode first.

[zhouhanc]

The problem is that health path is /health, which is missing a backslash. So when we attach a watcher to its child, the path becomes /healthchild1, which should be /health/child1.

I fixed the problem by appending a backslash after /health