Minor fixes to NamespaceMapping

[ctubbsii]

• Add a check to make sure that if the name exists in the mapping, then it results in a namespace exists exception, rather than only do that if the ID exists in the mapping (this fixes the NamespaceIT failure seen after Add ZooKeeper Namespace Id-to-name mapping #4996 was merged)
• Make NamespaceMapping.put() idempotent by checking the current mapping against the desired mapping (this preserves existing behavior at this point in the code, which was previously being handled by the Utils method that was deleted)

Trivial changes:

• Add some stricter checks for parameters to NamespaceMapping methods (final and require non-null)
• Avoid NPE problems by comparing the known non-null string on the left side of the .equals()
• Make name and inline comments consistent between rename and put methods