yang-data-impl: decrease memory overhead of wrapped maps

Profiling of a large-ish dataset loaded into the datastore has shown that we waste quite a bit of memory on UnmodifiableMap instances. As it turns out, our use of them leaves two out of three internal fields unused, which translates to pure overhead of 8 bytes per instance (in keySet and entrySet fields).

Create a minimal wrapper class, which will be defensive enough, which will only cache the values() field.