Optimize listing with leaf check offloaded to posix (#7541)

Other listing optimizations include

- remove double sorting while filtering object entries
- improve error message when upload-id is not in quorum
- use jsoniter for full unmarshal json, instead of gjson
- remove unused code

cmd/xl-v1-metadata.go

@@ -1,5 +1,5 @@
 /*
- * MinIO Cloud Storage, (C) 2016, 2017, 2017 MinIO, Inc.
+ * MinIO Cloud Storage, (C) 2016-2019 MinIO, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,6 +27,7 @@ import (
 	"sync"
 	"time"
 
+	jsoniter "github.com/json-iterator/go"
 	"github.com/minio/minio/cmd/logger"
 )
 
@@ -74,8 +75,8 @@ func (c ChecksumInfo) MarshalJSON() ([]byte, error) {
 
 // UnmarshalJSON - should never be called, instead xlMetaV1UnmarshalJSON() should be used.
 func (c *ChecksumInfo) UnmarshalJSON(data []byte) error {
-	logger.LogIf(context.Background(), errUnexpected)
 	var info checksumInfoJSON
+	var json = jsoniter.ConfigCompatibleWithStandardLibrary
 	if err := json.Unmarshal(data, &info); err != nil {
 		return err
 	}