Upgrade to futures 0.3.15.

Cargo.toml

@@ -13,8 +13,8 @@ keywords = ["raft", "consensus-protocol"]
 bincode = "1.3.3"
 bytes = "1.0"
 crossbeam-utils = "0.8"
-futures-channel = "0.3.8"
-futures-util = "0.3.8"
+futures-channel = "0.3.15"
+futures-util = "0.3.15"
 labrpc = { path = "../labrpc" }
 parking_lot = "0.11.1"
 rand = "0.8"
@@ -24,7 +24,7 @@ tokio = { version = "1.7", features = ["rt-multi-thread", "time", "parking_lot"]
 
 [dev-dependencies]
 anyhow = "1.0"
-futures = { version = "0.3.8", features = ["thread-pool"] }
+futures = { version = "0.3.15", features = ["thread-pool"] }
 scopeguard = "1.1.0"
 kvraft = { path = "kvraft" }