Limit the set of known extrinsics and blocks. (#1842)

* Limit the set of known extrinsics and blocks.

* Update Cargo.lock

* Always insert and pop oldest entry if needed.

Always inserting first ensures that the element's LRU position is
updated.

substrate/core/network/src/lib.rs

@@ -27,6 +27,7 @@ mod protocol;
 mod chain;
 mod blocks;
 mod on_demand;
+mod util;
 pub mod config;
 pub mod consensus_gossip;
 pub mod error;