Updates the message-id according to the Networking Spec (#1752)

## Proposed Changes

Implement the new message id function (see https://github.com/ethereum/eth2.0-specs/pull/2089) using an additional fast message id function for better performance + caching decompressed data.

beacon_node/eth2_libp2p/src/behaviour/handler/delegate.rs

@@ -1,8 +1,8 @@
+use crate::behaviour::Gossipsub;
 use crate::rpc::*;
 use libp2p::{
     core::either::{EitherError, EitherOutput},
     core::upgrade::{EitherUpgrade, InboundUpgrade, OutboundUpgrade, SelectUpgrade, UpgradeError},
-    gossipsub::Gossipsub,
     identify::Identify,
     swarm::{
         protocols_handler::{