Define "next state root hash" for `BlockChain` and its behavior and refactor relevant code.

[greymistcube]

We only need single "next state root hash" at any time for a BlockChain object and this does not need to be tracked in storage. As for BlockChain.GetNextStateRootHash() method, there are numerous problems:

• It isn't at all clear how the internal dictionary is managed. The result is too reliant code-paths that can be taken.
  • Having management of its state in the background only obfuscates it being rather ill-defined, which exacerbates the issue.
• The method has a blocking implementation. This shouldn't be blocked from BlockChain's side, but rather the caller's side.
• Shared storage between possible forks, although this may not be a non-issue in practice, adds unnecessary complexity.