Fix health race (#645)

* Revert "middleware/proxy: Make Unhealthy a pointer (#615)"

This reverts commit acbf522ceb.

* middleware/proxy: add proper locking

This add the proper locking around `Unhealthy`.

middleware/proxy/policy_test.go

@@ -28,16 +28,13 @@ func (r *customPolicy) Select(pool HostPool) *UpstreamHost {
 func testPool() HostPool {
 	pool := []*UpstreamHost{
 		{
-			Name:      workableServer.URL, // this should resolve (healthcheck test)
-			Unhealthy: newBool(),
+			Name: workableServer.URL, // this should resolve (healthcheck test)
 		},
 		{
-			Name:      "http://shouldnot.resolve", // this shouldn't
-			Unhealthy: newBool(),
+			Name: "http://shouldnot.resolve", // this shouldn't
 		},
 		{
-			Name:      "http://C",
-			Unhealthy: newBool(),
+			Name: "http://C",
 		},
 	}
 	return HostPool(pool)
@@ -57,7 +54,7 @@ func TestRoundRobinPolicy(t *testing.T) {
 		t.Error("Expected second round robin host to be third host in the pool.")
 	}
 	// mark host as down
-	*pool[0].Unhealthy = true
+	pool[0].Unhealthy = true
 	h = rrPolicy.Select(pool)
 	if h != pool[1] {
 		t.Error("Expected third round robin host to be first host in the pool.")