Create a custom http.Client for apns communication (#446)

In order to set custom transport options on the http.Client used
by the push service we need to create our own instance. This
allows us to configure the IdleConnTimeout and prevent connections
from being killed and pushes not being sent. 90 seconds is used
here since it's the same as the DefaultTransport provided by Go.

platform/apns/service.go

@@ -5,10 +5,13 @@ import (
 	"crypto/tls"
 	"fmt"
 	"log"
+	"net/http"
 	"sync"
+	"time"
 
 	"github.com/RobotsAndPencils/buford/push"
 	"github.com/pkg/errors"
+	"golang.org/x/net/http2"
 
 	"github.com/micromdm/micromdm/platform/config"
 	"github.com/micromdm/micromdm/platform/pubsub"
@@ -121,13 +124,33 @@ func updateClient(svc *PushService, sub pubsub.Subscriber) error {
 	return nil
 }
 
+func newClient(cert tls.Certificate) (*http.Client, error) {
+	config := &tls.Config{
+		Certificates: []tls.Certificate{cert},
+	}
+	config.BuildNameToCertificate()
+	transport := &http.Transport{
+		TLSClientConfig: config,
+		IdleConnTimeout: 90 * time.Second,
+	}
+
+	if err := http2.ConfigureTransport(transport); err != nil {
+		return nil, err
+	}
+
+	return &http.Client{
+		Transport: transport,
+		Timeout:   20 * time.Second,
+	}, nil
+}
+
 func NewPushService(provider PushCertificateProvider) (*push.Service, error) {
 	cert, err := provider.PushCertificate()
 	if err != nil {
 		return nil, errors.Wrap(err, "get push certificate from store")
 	}
 
-	client, err := push.NewClient(*cert)
+	client, err := newClient(*cert)
 	if err != nil {
 		return nil, errors.Wrap(err, "create push service client")
 	}