Begin to unify ResourceVersioner and SelfLinker

Create a new MetadataAccessor interface that combines both
and use it where previously latest.ResourceVersioner and SelfLinker
were being used.

Adds Namespace to the get/set interface. Adds TODO about future
fast path for metadata (as per thockin's comment)

pkg/tools/etcd_tools_test.go

@@ -46,7 +46,7 @@ func (*TestResource) IsAnAPIObject() {}
 
 var scheme *runtime.Scheme
 var codec runtime.Codec
-var versioner = RuntimeVersionAdapter{meta.NewResourceVersioner()}
+var versioner = RuntimeVersionAdapter{meta.NewAccessor()}
 
 func init() {
 	scheme = runtime.NewScheme()