Snapshot state to a genesis file

[carver]

What was wrong?

Inspired by #88 (comment)

People want to pick up chain state from somewhere they left off in a test chain. It would be cool to use this to set up integration tests for non-eth-tester nodes too!

How can it be fixed?

It would be sweet to have an easy way to snapshot state into a genesis file (which can then be reimported using #129).

I think a good API is: genesis_dict = tester.genesis_snapshot(). First step is consensus on that.

[pipermerriam]

I suspect we should also be able to use an extended version of EIP1085 for this, with something like an additional blocks key.

[evertonfraga]

With the risk of being too late in the game, I ask: how should contract accounts' state be represented in a "non zero-block" snapshot?

[carver]

I've been defaulting to this (unmerged) proposal for genesis file format: ethereum/EIPs#1085

So you would list the storage slots. in each contract account.

Probably the most work for implementing this is that we do not currently track preimages (of slots or account addresses). So we can't just iterate the database to create the state, we have to add a preimage tracker, I think. Maybe this belongs as a custom AccountDB and StorageDB, but I'm not sure without diving in to design it.