Store state trie in separate DB and only update relevant branches

[arjanz]

For performance reasons and when state does not fit into memory anymore, we need to store the state trie in a separate DB and only partially update the trie.

Todo since #96

[arjanz]

Stub code: https://gist.github.com/arjanz/dd575f757baa09bc7a38143743da4e4d