Add support for multi-nonce

[danfinlay]

Multi-nonce is a strategy of replay protection that allows parallel queues of transactions. It was described in some detail by PISA research (@stonecoldpat, @yahgwai) in this article and hackathon entry.

They have a working implementation here.

In multi-nonce, there are two dimensions to a nonce:
- Row
- Transaction Number

Each row’s transactions must be executed in order, but there may be an arbitrary number of rows, allowing highly parallelized transaction queues.

This can solve one of the most annoying behaviors of nonce-based accounts: Having high-value transactions stuck by low-value transactions. It can also allow the creation and sharing of meta-transactions that can be redeemed in any order, which I think could allow for very exciting wallet design that minimizes on-chain transactions until necessary.

I know this would be a pretty fundamental change, so isn't a shoe-in, but I wanted to share this desire so it could be considered.

[rmeissner]

Hey this seems to be duplicate of #199 with the additoon of the information on the (very interesting) use case.

[rmeissner]

Duplicate of #199