decrypt ETags in parallel around 500 at a time (#10261)

Listing speed-up gained from 10secs for
just 400 entries to 2secs for 400 entries

cmd/crypto/header.go

@@ -18,10 +18,10 @@ import (
 	"bytes"
 	"crypto/md5"
 	"encoding/base64"
-	"encoding/json"
 	"net/http"
 	"strings"
 
+	jsoniter "github.com/json-iterator/go"
 	xhttp "github.com/minio/minio/cmd/http"
 )
 
@@ -148,6 +148,7 @@ func (s3KMS) ParseHTTP(h http.Header) (string, interface{}, error) {
 	contextStr, ok := h[SSEKmsContext]
 	if ok {
 		var context map[string]interface{}
+		var json = jsoniter.ConfigCompatibleWithStandardLibrary
 		if err := json.Unmarshal([]byte(contextStr[0]), &context); err != nil {
 			return "", nil, err
 		}