* Check whether `HashKey` can be reduced to a simple 32 byte array (see
  *desc_identifiers.nim*)

* Remove the `RlpData` accounts payload type. It is not needed as a separate
  data type. An application must know the layout. So it can be subsumed
  under `RawData` (which could be renamed `PlainData`.)

* Currently, the data save/store logic only works when there is s VertexID(1)
  root. In tests without a `VertexID(1)` a dummy vertex is set up.

* Re-visit `delTree()`. Suggestion is deleting small trees on the memory later,
  otherwise only deleting the root vertex (so it becomes inaccessible) and
  remember the follow up vertices which can travel through the tx-layers
  to be picked up by the backend store.