Address ENR update loop (#2216)

## Issue Addressed

- Resolves #2215

## Proposed Changes

Addresses a potential loop when the majority of peers indicate that we are contactable via an IPv6 address.

See https://github.com/sigp/discv5/pull/62 for further rationale.

## Additional Info

The alternative to this PR is to use `--disable-enr-auto-update` and then manually supply an `--enr-address` and `--enr-upd-port`. However, that requires the user to know their IP addresses in order for discovery to work properly. This might not be practical/achievable for some users, hence this hotfix.

beacon_node/eth2_libp2p/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["Sigma Prime <contact@sigmaprime.io>"]
 edition = "2018"
 
 [dependencies]
-discv5 = { version = "0.1.0-beta.3", features = ["libp2p"] }
+discv5 = { git = "https://github.com/sigp/discv5 ", rev = "02d2c896c66f8dc2b848c3996fedcd98e1dfec69", features = ["libp2p"] }
 unsigned-varint = { version = "0.6.0", features = ["codec"] }
 types = { path =  "../../consensus/types" }
 hashset_delay = { path = "../../common/hashset_delay" }

beacon_node/http_api/Cargo.toml

@@ -34,4 +34,4 @@ futures = "0.3.8"
 store = { path = "../store" }
 environment = { path = "../../lighthouse/environment" }
 tree_hash = "0.1.1"
-discv5 = { version = "0.1.0-beta.3" }
+discv5 = { git = "https://github.com/sigp/discv5 ", rev = "02d2c896c66f8dc2b848c3996fedcd98e1dfec69", features = ["libp2p"] }