Add caching to block_header()

ethcore/light/src/client/header_chain.rs

@@ -365,6 +365,14 @@ impl HeaderChain {
 	/// will be returned.
 	pub fn block_header(&self, id: BlockId) -> Option<encoded::Header> {
 		let load_from_db = |hash: H256| {
+			let cached = {
+				let mut cache = self.cache.lock();
+				cache.block_header(&hash)
+			};
+
+			if cached.is_some() {
+				return cached
+			} else {
 				match self.db.get(self.col, &hash) {
 					Ok(val) => val.map(|x| x.to_vec()).map(encoded::Header::new),
 					Err(e) => {
@@ -372,6 +380,7 @@ impl HeaderChain {
 						None
 					}
 				}
+			}
 		};
 
 		match id {