Update dependency crossbeam-utils to the newest version.

Cargo.toml

@@ -12,7 +12,7 @@ keywords = ["raft", "consensus-protocol"]
 [dependencies]
 bincode = "1.3.1"
 bytes = "0.5.6"
-crossbeam-utils = "0.7.2"
+crossbeam-utils = "0.8"
 futures = { version = "0.3.5" }
 labrpc = { path = "../labrpc" }
 parking_lot = "0.11.0"