Update discv5 to v10 (#1592)

## Issue Addressed

Code improvements, dependency improvements and better async handling.

Cargo.lock

@@ -1213,10 +1213,11 @@ checksum = "212d0f5754cb6769937f4501cc0e67f4f4483c8d2c3e1e922ee9edbe4ab4c7c0"
 
 [[package]]
 name = "discv5"
-version = "0.1.0-alpha.8"
+version = "0.1.0-alpha.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "90782d49541b01f9b7e34e6af5d80d01396bf7b1a81505a0035da224134b8d73"
+checksum = "b4cba1b485c16864edc11ccbf3abf5fbf1c26ce759ab36c32ee8e12638d50b0d"
 dependencies = [
+ "aes-gcm",
  "arrayvec",
  "digest 0.8.1",
  "enr",
@@ -1231,7 +1232,6 @@ dependencies = [
  "lru_time_cache",
  "multihash",
  "net2",
- "openssl",
  "parking_lot 0.11.0",
  "rand 0.7.3",
  "rlp",

beacon_node/eth2_libp2p/Cargo.toml

@@ -32,7 +32,7 @@ snap = "1.0.0"
 void = "1.0.2"
 tokio-io-timeout = "0.4.0"
 tokio-util = { version = "0.3.1", features = ["codec", "compat"] }
-discv5 = { version = "0.1.0-alpha.8", features = ["libp2p", "openssl-vendored"] }
+discv5 = { version = "0.1.0-alpha.10", features = ["libp2p"] }
 tiny-keccak = "2.0.2"
 environment = { path = "../../lighthouse/environment" }
 # TODO: Remove rand crate for mainnet

lighthouse/environment/Cargo.toml

@@ -21,4 +21,4 @@ slog-json = "2.3.0"
 exit-future = "0.2.0"
 lazy_static = "1.4.0"
 lighthouse_metrics = { path = "../../common/lighthouse_metrics" }
-discv5 = { version = "0.1.0-alpha.8", features = ["libp2p", "openssl-vendored"] }
+discv5 = { version = "0.1.0-alpha.10", features = ["libp2p"] }