build(deps): update tokio-util to v0.7

This branch updates `tokio-util` to v0.7.0.

The dependabot-generated PR for this update doesn't work, because this
is a breaking API change. The only update that was necessary here was
changing the use of `tokio_util::sync::PollSender` in
`linkerd-proxy-discover` to use the updated method names.

Closes #240

Cargo.lock

@@ -457,7 +457,7 @@ dependencies = [
  "indexmap",
  "slab",
  "tokio",
- "tokio-util",
+ "tokio-util 0.6.9",
  "tracing",
 ]
 
@@ -1169,7 +1169,7 @@ dependencies = [
  "pin-project",
  "tokio",
  "tokio-test",
- "tokio-util",
+ "tokio-util 0.7.0",
 ]
 
 [[package]]
@@ -1304,7 +1304,7 @@ dependencies = [
  "linkerd-stack",
  "pin-project",
  "tokio",
- "tokio-util",
+ "tokio-util 0.6.9",
  "tower",
  "tracing",
 ]
@@ -2460,7 +2460,7 @@ dependencies = [
  "futures-core",
  "pin-project-lite",
  "tokio",
- "tokio-util",
+ "tokio-util 0.6.9",
 ]
 
 [[package]]
@@ -2490,6 +2490,20 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tokio-util"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "64910e1b9c1901aaf5375561e35b9c057d95ff41a44ede043a03e09279eabaf1"
+dependencies = [
+ "bytes",
+ "futures-core",
+ "futures-sink",
+ "log",
+ "pin-project-lite",
+ "tokio",
+]
+
 [[package]]
 name = "tonic"
 version = "0.6.2"
@@ -2513,7 +2527,7 @@ dependencies = [
  "prost-derive",
  "tokio",
  "tokio-stream",
- "tokio-util",
+ "tokio-util 0.6.9",
  "tower",
  "tower-layer",
  "tower-service",
@@ -2548,7 +2562,7 @@ dependencies = [
  "slab",
  "tokio",
  "tokio-stream",
- "tokio-util",
+ "tokio-util 0.6.9",
  "tower-layer",
  "tower-service",
  "tracing",

linkerd/io/Cargo.toml

@@ -19,7 +19,7 @@ bytes = "1"
 linkerd-errno = { path = "../errno" }
 tokio = { version = "1", features = ["io-util", "net"] }
 tokio-test = { version = "0.4", optional = true }
-tokio-util = { version = "0.6", features = ["io"] }
+tokio-util = { version = "0.7", features = ["io"] }
 pin-project = "1"
 
 [dev-dependencies]

linkerd/proxy/discover/Cargo.toml

@@ -16,7 +16,7 @@ linkerd-error = { path = "../../error" }
 linkerd-proxy-core = { path = "../core" }
 linkerd-stack = { path = "../../stack" }
 tokio = { version = "1", features = ["rt", "sync", "time"] }
-tokio-util = "0.6.9"
+tokio-util = "0.7"
 tower = { version = "0.4.11", features = ["discover"] }
 tracing = "0.1.30"
 pin-project = "1"