DatanodeInfo getNetworkLocation and setNetworkLocation shoud use volatile instead of synchronized

DatanodeInfo has synchronized methods getNetworkLocation and setNetworkLocation. While they doing nothing more than setting and getting variable location.

Since location is not being modified based on its current value and is independent from any other variables. This JIRA propose to remove synchronized methods but only make location volatile. Such that threads will not be blocked on get/setNetworkLocation.

Thanks Tsz-wo Sze for the offline disscussion.