re-use transport for AdminInfo() call (#14571)

avoids creating new transport for each `isServerResolvable`
request, instead re-use the available global transport and do
not try to forcibly close connections to avoid TIME_WAIT
build upon large clusters.

Never use httpClient.CloseIdleConnections() since that can have
a drastic effect on existing connections on the transport pool.

Remove it everywhere.

cmd/admin-handlers.go

@@ -21,7 +21,6 @@ import (
 	"context"
 	crand "crypto/rand"
 	"crypto/subtle"
-	"crypto/tls"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -2332,19 +2331,9 @@ func checkConnection(endpointStr string, timeout time.Duration) error {
 	ctx, cancel := context.WithTimeout(GlobalContext, timeout)
 	defer cancel()
 
-	client := &http.Client{Transport: &http.Transport{
-		Proxy:                 http.ProxyFromEnvironment,
-		DialContext:           xhttp.NewCustomDialContext(timeout),
-		ResponseHeaderTimeout: 5 * time.Second,
-		TLSHandshakeTimeout:   5 * time.Second,
-		ExpectContinueTimeout: 5 * time.Second,
-		TLSClientConfig:       &tls.Config{RootCAs: globalRootCAs},
-		// Go net/http automatically unzip if content-type is
-		// gzip disable this feature, as we are always interested
-		// in raw stream.
-		DisableCompression: true,
-	}}
-	defer client.CloseIdleConnections()
+	client := &http.Client{
+		Transport: globalProxyTransport,
+	}
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodHead, endpointStr, nil)
 	if err != nil {