Dynamically adjust PIP request costs based on gathered data (#5603)

* beginnings of load timer

* initial load timer implementation

* saturating adds

* create flow params from distribution

* update request credits and acknowledgement

* mark cumulative cost dead code

* fix compilation

* tests

* supply load share and other params to  lightprotocol params

* add file store

* fix ethsync compilation

* reshuffle constants

ethcore/light/src/net/request_set.rs

@@ -120,6 +120,8 @@ impl RequestSet {
 	pub fn is_empty(&self) -> bool { self.len() == 0 }
 
 	/// The cumulative cost of all requests in the set.
+	// this may be useful later for load balancing.
+	#[allow(dead_code)]
 	pub fn cumulative_cost(&self) -> U256 { self.cumulative_cost }
 }