Remove Sparse Merkle tree usage from the ContractsState and ContractsAssets tables

[xgreenx]

Overview

Based on the latest research that we did, there exist other Merkle trees that show better performance, less touch the storage, and produce less footprint.

Because of that, we've decided to use a tree other than SMT. But their production-ready audited implementation will take some time. Until that moment, we just need to remove usage of SMT.

Implementation details

Use a Plain blueprint instead of the Sparse for ContractsState and ContractsAssets tables.

Since the Plain blueprint doesn't implement MerkleRootStorage, in the places where we used root, we need to use and root of the empty tree.