Proper light client informant and more verification of imported headers (#5897)

* do more validation of imported headers in light client

* generalize informant with traits

* informant implementation for light client

* make comment into TODO

* fix broken test

* disable full checking of headers in light client in sync tests

ethcore/light/src/cache.rs

@@ -26,7 +26,7 @@ use ethcore::receipt::Receipt;
 
 use stats::Corpus;
 use time::{SteadyTime, Duration};
-use util::{U256, H256};
+use util::{U256, H256, HeapSizeOf};
 use util::cache::MemoryLruCache;
 
 /// Configuration for how much data to cache.
@@ -153,6 +153,22 @@ impl Cache {
 	pub fn set_gas_price_corpus(&mut self, corpus: Corpus<U256>) {
 		self.corpus = Some((corpus, SteadyTime::now()))
 	}
+
+	/// Get the memory used.
+	pub fn mem_used(&self) -> usize {
+		self.heap_size_of_children()
+	}
+}
+
+impl HeapSizeOf for Cache {
+	fn heap_size_of_children(&self) -> usize {
+		self.headers.current_size()
+			+ self.canon_hashes.current_size()
+			+ self.bodies.current_size()
+			+ self.receipts.current_size()
+			+ self.chain_score.current_size()
+			// TODO: + corpus
+	}
 }
 
 #[cfg(test)]