details:
Persistent pruning would not restore the `emptyRlp` value for the
root node when the database becomes empty. This effects to an
assertion exception next time the DB is accessed.
As most unit tests run on the memory DB, this case slipped through
unnoticed for a while (see also issue #9.)
* Aristo: remove obsolete functions
* Aristo: Fix error code for non-available hash keys
why:
Must not return `not-found` when the key is not available (i.e. the
current changes were not hashified, yet.)
* CoreDB: Provide TDD and test framework