Market-orientated transaction pricing (openethereum#1963)

* Market-orientated transaction pricing

Avoid a strict gas-limit and let the market decide through using
a priority queue based around gas pricing for transactions. In
periods of low transaction volume, they'll be processed for a lower
fee.

* Fix tests, add/clarify documentation, fix some logic.

* Change default to reflect CLI.

* Specify type.

* Make test more precise.

* Fix doc test