Get connected host and port of ZooKeeper 

I implemented `get_connected_host_and_port` method in `lock_service`.
This is the utility method that can get  host and port of ZooKeeper connected.

I think it is useful for `get_status` or printing error message.

This include the patch for https://github.com/jubatus/jubatus/issues/574 .
I added log message as follows:
```
I1210 11:01:49.510366 19570 zk.cpp:91] connected to zk: 127.0.0.1:2201
```

[NOTE]
* This method is not support IPv6 environment.