use signature-v2 for 'object perf' tests to avoid CPU using sha256 (#15151)

It is observed in a local 8 drive system the CPU seems to be
bottlenecked at

```
(pprof) top
Showing nodes accounting for 1385.31s, 88.47% of 1565.88s total
Dropped 1304 nodes (cum <= 7.83s)
Showing top 10 nodes out of 159
      flat  flat%   sum%        cum   cum%
      724s 46.24% 46.24%       724s 46.24%  crypto/sha256.block
   219.04s 13.99% 60.22%    226.63s 14.47%  syscall.Syscall
   158.04s 10.09% 70.32%    158.04s 10.09%  runtime.memmove
   127.58s  8.15% 78.46%    127.58s  8.15%  crypto/md5.block
    58.67s  3.75% 82.21%     58.67s  3.75%  github.com/minio/highwayhash.updateAVX2
    40.07s  2.56% 84.77%     40.07s  2.56%  runtime.epollwait
    33.76s  2.16% 86.93%     33.76s  2.16%  github.com/klauspost/reedsolomon._galMulAVX512Parallel84
     8.88s  0.57% 87.49%     11.56s  0.74%  runtime.step
     7.84s   0.5% 87.99%      7.84s   0.5%  runtime.memclrNoHeapPointers
     7.43s  0.47% 88.47%     22.18s  1.42%  runtime.pcvalue
```

Bonus changes:

- re-use transport for bucket replication clients, also site replication clients.
- use 32KiB buffer for all read and writes at transport layer seems to help
  TLS read connections.
- Do not have 'MaxConnsPerHost' this is problematic to be used with net/http
  connection pooling 'MaxIdleConnsPerHost' is enough.

cmd/site-replication.go

@@ -2094,7 +2094,7 @@ func getAdminClient(endpoint, accessKey, secretKey string) (*madmin.AdminClient,
 	if err != nil {
 		return nil, err
 	}
-	client.SetCustomTransport(NewRemoteTargetHTTPTransport())
+	client.SetCustomTransport(globalRemoteTargetTransport)
 	return client, nil
 }
 
@@ -2106,7 +2106,7 @@ func getS3Client(pc madmin.PeerSite) (*minioClient.Client, error) {
 	return minioClient.New(ep.Host, &minioClient.Options{
 		Creds:     credentials.NewStaticV4(pc.AccessKey, pc.SecretKey, ""),
 		Secure:    ep.Scheme == "https",
-		Transport: NewRemoteTargetHTTPTransport(),
+		Transport: globalRemoteTargetTransport,
 	})
 }