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/bucket-targets.go

@@ -19,7 +19,6 @@ package cmd
 
 import (
 	"context"
-	"net/http"
 	"sync"
 	"time"
 
@@ -325,25 +324,16 @@ func (sys *BucketTargetSys) set(bucket BucketInfo, meta BucketMetadata) {
 	sys.targetsMap[bucket.Name] = cfg.Targets
 }
 
-// getRemoteTargetInstanceTransport contains a singleton roundtripper.
-var (
-	getRemoteTargetInstanceTransport     http.RoundTripper
-	getRemoteTargetInstanceTransportOnce sync.Once
-)
-
 // Returns a minio-go Client configured to access remote host described in replication target config.
 func (sys *BucketTargetSys) getRemoteTargetClient(tcfg *madmin.BucketTarget) (*TargetClient, error) {
 	config := tcfg.Credentials
 	creds := credentials.NewStaticV4(config.AccessKey, config.SecretKey, "")
-	getRemoteTargetInstanceTransportOnce.Do(func() {
-		getRemoteTargetInstanceTransport = NewRemoteTargetHTTPTransport()
-	})
 
 	api, err := minio.New(tcfg.Endpoint, &miniogo.Options{
 		Creds:     creds,
 		Secure:    tcfg.Secure,
 		Region:    tcfg.Region,
-		Transport: getRemoteTargetInstanceTransport,
+		Transport: globalRemoteTargetTransport,
 	})
 	if err != nil {
 		return nil, err