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/eth2_libp2p/Cargo.toml

@@ -41,7 +41,7 @@ rand = "0.7.3"
 [dependencies.libp2p]
 #version = "0.19.1"
 git = "https://github.com/sigp/rust-libp2p"
-rev = "147bb43fa56c1b84253606eabedb0794eeed8b94"
+rev = "8e9e35994e63716c6eb0a05b9702133d113b3822"
 default-features = false
 features = ["websocket", "identify", "mplex", "yamux", "noise", "gossipsub", "dns", "secio", "tcp-tokio"]