Make `NetworkGraph` Clone-able again

Make `NetworkGraph` Clone-able again

There isn't a lot of user-utility for cloning `NetworkGraph`
directly (its a rather large struct, and there probably isn't a lot
of reason to have *multiple* `NetworkGraph`s). Thus, when locks
were pushed down into it, the `Clone`-ability of it was dropped as
well.

Sadly, mapping the Java memory model onto:
 * `Read`-ing a `NetworkGraph`, creating a Java-owned
   `NetworkGraph` object that the JVM will destruct for us,
 * Passing it to a `NetGraphMsgHandler`, which now expects to own
   the `NetworkGraph`, including destructing it,
isn't really practical without adding a clone in between.

Given this, and the fact that there's nothing inherently wrong with
clone-ing a `NetworkGraph`, we simply re-add `Clone` here.