Automatic transaction repricing / replacement

[iamdefinitelyahuman]

What I did

Automatic repricing of transactions.

Closes #727

How I did it

When setting gas_price, you may now use an object that subclasses from GasABC:

• SimpleGasStrategy for simple strategies that are called once
• BlockGasStrategy which is called every n blocks
• TimeGasStrategy which is called every n seconds

For strategies that trigger tx replacement, returning a non-None value broadcasts a new transaction.

Thus far I have created two actual strategies, based around the gasnow API.

How to verify it

Try it out!

TODO

• more strategies
• consideration / updates to logic re: console output while waiting for a tx to confirm
• detailed natspec for strategies and ABCs
• documentation
• unit testing (not sure how this will look)

[iamdefinitelyahuman]

Still need to add another strategy or two here, but this is close!

cc @banteg @fubuloubu

[banteg]

Another potentially useful default strategy for Geth with graphql enabled:

def gas_price(position=500):
    query = "{ pending { transactions { gasPrice }}}"
    resp = requests.post(f"{ETH_RPC_URL}/graphql", json={"query": query})
    data = resp.json()["data"]["pending"]["transactions"]
    prices = [int(x["gasPrice"], 16) for x in data]
    return Wei(sorted(prices, reverse=True)[:position][-1])

[iamdefinitelyahuman]

This should be gtg now

[fubuloubu]

Gave it a shot, looks pretty good to me.