Consider implementing GetStorageHash method

[kjezek]

At the moment, we do not provide this method, and provide only HasEmptyStotage method that returns bool flag saying if the storage is empty or not.

GetStorageHash returns real Hash of the storge root.

Originally we thought that it is performance prohibitive to allow for computing the storage hash at any time. But Ethereum assures this value only for sealed blocks, and for intermediate results it uses statedb temporal values. We do it with the HasEmptyStorage the same anyway, i.e. it would have been no harm to return the hash and implement original GetStorageHash.