Refactored a lot of State.
Killed playback() API.
State diffing at arbitrary places in chain.
Debug past transactions in debugger.
Fixes for state debugging.
Merge branch 'develop' of github.com:ethereum/cpp-ethereum into develop

* 'develop' of github.com:ethereum/cpp-ethereum:
  Fixes for state debugging.
  Debug past transactions in debugger.
  Refactored a lot of State. Killed playback() API. State diffing at arbitrary places in chain.