BUG 1597 - Do not use toString as serialized version of YangInstanceIdentifier/PathArgument

The code has been modified to stop interpreting YangInstanceIdentifier.toString
and PathArgument.toString as a serialized versions of those objects. The toString
implementations of those objects has now been moved into PathUtils.

- NormalizedNodeGetter and NormalizedNodeToNodeCodecTest have been modified to use
PathUtils.toString instead of YangInstanceIdentifier.toString or PathArgument.toString

- Added tests to cover all the utilities in PathUtils

Change-Id: I1ed2c452e15a45229c36532d59db7fa1a77d283d
Signed-off-by: Moiz Raja <moraja@cisco.com>
Merge "BUG 1597 - Do not use toString as serialized version of YangInstanceIdentifier/PathArgument"