Market-orientated transaction pricing (#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

parity/cli.rs

@@ -170,7 +170,7 @@ Sealing/Mining Options:
                            lenient - Same as strict when mining, and cheap
                            when not [default: cheap].
   --usd-per-tx USD         Amount of USD to be paid for a basic transaction
-                           [default: 0.005]. The minimum gas price is set
+                           [default: 0]. The minimum gas price is set
                            accordingly.
   --usd-per-eth SOURCE     USD value of a single ETH. SOURCE may be either an
                            amount in USD, a web service or 'auto' to use each

parity/params.rs

@@ -179,7 +179,7 @@ pub enum GasPricerConfig {
 impl Default for GasPricerConfig {
 	fn default() -> Self {
 		GasPricerConfig::Calibrated {
-			usd_per_tx: 0.005,
+			usd_per_tx: 0f32,
 			recalibration_period: Duration::from_secs(3600),
 		}
 	}