This method returns persisted block height and doesn't take into account
persisting block height. Some of the callers of this method relay on
the wrong assumption that BlockHeight() returns persisting block index.
Fix improper usages of this method and adjust tests. Ref.
61a066583e/src/Neo/SmartContract/ApplicationEngine.cs (L634).
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>