Gossipsub update (#1400)

## Issue Addressed

N/A

## Proposed Changes

This provides a number of corrections and improvements to gossipsub. Specifically
- Enables options for greater privacy around the message author
- Provides greater flexibility on message validation
- Prevents unvalidated messages from being gossiped
- Shifts the duplicate cache to a time-based cache inside gossipsub
- Updates the message-id to handle bytes
- Bug fixes related to mesh maintenance and topic subscription. This should improve our attestation inclusion rate.

beacon_node/network/src/router/mod.rs

@@ -297,10 +297,10 @@ impl<T: BeaconChainTypes> Router<T> {
         }
     }
 
-    /// Informs the network service that the message should be forwarded to other peers.
+    /// Informs the network service that the message should be forwarded to other peers (is valid).
     fn propagate_message(&mut self, message_id: MessageId, propagation_source: PeerId) {
         self.network_send
-            .send(NetworkMessage::Propagate {
+            .send(NetworkMessage::Validate {
                 propagation_source,
                 message_id,
             })