WIP: Refactory how account db is used.

[pipermerriam]

link: #599

What was wrong?

This is an attempt at an incremental first step towards #599. It also ended up doing some bits of what I think relate to #507

• During the initial implementaiton of the sharding code, support for multiple trie types was added. Changes to the sharding roadmap removed the need for supporting multiple tries.
• The name AccountStateDB is more concisely named AccountDB
• The name account_db is more appropriate than state_db
• The State class is a more appropriate home for the the account_db
• The context manager APIs aren't working out in my opinion and are low value add.

How was it fixed?

This is a work in progress. I'm trying to simplify the API through which we access and modify the account state as well as figure out a better mechanism for updating the state_root.

• TODO

Cute Animal Picture

[carver]

first step towards #599. It also ended up doing some bits of what I think relate to #507

called it :P