MM-23460: Reuse a single HTTP client across all push notifications (#14122)

Before this, for every single request we would create a new http.Client
from scratch. This was costly because every new client has internal
TCP connections which it reuses.

Therefore, originally, beyond a throughput of 800-1200 rps with a
concurrency of 50, beyond which it would run out of file descriptors.

Now it can support upto 8000-10000 rps.
An improvement by a factor of 10.

Throughput numbers:
BenchmarkPushNotification
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 11445.091601 reqs/s
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 9190.761518 reqs/s
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 8334.394764 reqs/s
BenchmarkPushNotification-8   	     219	   5999304 ns/op	 3369224 B/op	   42127 allocs/op
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 9590.399780 reqs/s
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 9347.137463 reqs/s
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 9053.784250 reqs/s
BenchmarkPushNotification-8   	     223	   5522636 ns/op	 3368470 B/op	   42034 allocs/op
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 10250.783365 reqs/s
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 9123.292899 reqs/s
    BenchmarkPushNotification: notification_push_test.go:1478: throughput: 9173.553180 reqs/s

This is the aggregate count using benchstat.
The number is for a single run of sending 50 requests.

name                time/op
PushNotification-8  5.61ms ± 7%

name                alloc/op
PushNotification-8  3.36MB ± 1%

name                allocs/op
PushNotification-8   42.0k ± 1%

Tested on a machine with ulimit -n = 4096.

app/server.go

@@ -118,7 +118,8 @@ type Server struct {
 
 	phase2PermissionsMigrationComplete bool
 
-	HTTPService httpservice.HTTPService
+	HTTPService            httpservice.HTTPService
+	pushNotificationClient *http.Client // TODO: move this to it's own package
 
 	ImageProxy *imageproxy.ImageProxy
 
@@ -204,6 +205,7 @@ func NewServer(options ...Option) (*Server, error) {
 	})
 
 	s.HTTPService = httpservice.MakeHTTPService(s)
+	s.pushNotificationClient = s.HTTPService.MakeClient(true)
 
 	s.ImageProxy = imageproxy.MakeImageProxy(s, s.HTTPService, s.Log)