Gas price engine API

What was wrong?

Currently, web3.py just uses the eth_gasPrice JSON-RPC API to determine the gas price to use for transactions. Most nodes do not currently implement very good algorithms for determining gas prices.

How can it be fixed?

Lets implement a new API for determining gas prices.

• Add a new API: Web3.set_gas_price_engine(engine)
• Add new API: Web3.get_gas_price(transaction_params) which delegates to whatever engine is set.

An engine must be a callable with the function signature:

engine(w3, transaction_params)

• w3 will be the Web3 instance
• transaction_params will be a dictionary of the transaction params being sent.

The callable must return a positive integer which represents the gas_price in the wei denomination.

The default engine that a web3 instance should use is one that simply returns web3.eth.gasPrice

Documentation should be added covering the following topics.

• Overview of the Web3.set_gas_price_engine() API (maybe in docs/overview.rst)
• Writing your own gas price engine (maybe in new document docs/gas_pricing_engine.rst or docs/web3.main.rst)
• List of available engines, where to import them and how to use them.

[pipermerriam]

@monokh etherscan.io/tx/0x20718ee637c2c01dfab0c7c0e9747f50512ef934e94462f077ced545d8926683

[pipermerriam]

@monokh take a look at #496 which builds on top of the API suggested in this PR and is a simplified version of the EGS algorithm.