Simplify GraphImpl internals further

**Depends on #120. Incremental diff: https://github.com/ankurdave/graphx/compare/6d1bf0d...more-simplify**

* VertexPlacement is only used in VTableReplicated, so this PR creates it on demand there rather than passing it around in GraphImpl.
* Rename VTableReplicated to ReplicatedVertexView.

Closes #119.