[feat] use rename instead of recursive deletes (#11641)

most of the delete calls today spend time in
a blocking operation where multiple calls need
to be recursively sent to delete the objects,
instead we can use rename operation to atomically
move the objects from the namespace to `tmp/.trash`

we can schedule deletion of objects at this
location once in 15, 30mins and we can also add
wait times between each delete operation.

this allows us to make delete's faster as well
less chattier on the drives, each server runs locally
a groutine which would clean this up regularly.

cmd/object-api-putobject_test.go

@@ -340,8 +340,19 @@ func testObjectAPIPutObjectStaleFiles(obj ObjectLayer, instanceType string, disk
 
 	for _, disk := range disks {
 		tmpMetaDir := path.Join(disk, minioMetaTmpBucket)
-		if !isDirEmpty(tmpMetaDir) {
-			t.Fatalf("%s: expected: empty, got: non-empty", minioMetaTmpBucket)
+		files, err := ioutil.ReadDir(tmpMetaDir)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var found bool
+		for _, fi := range files {
+			if fi.Name() == ".trash" {
+				continue
+			}
+			found = true
+		}
+		if found {
+			t.Fatalf("%s: expected: empty, got: non-empty %#v", minioMetaTmpBucket, files)
 		}
 	}
 }
@@ -418,8 +429,17 @@ func testObjectAPIMultipartPutObjectStaleFiles(obj ObjectLayer, instanceType str
 			t.Errorf("%s", err)
 		}
 
-		if len(files) != 0 {
-			t.Fatalf("%s: expected: empty, got: non-empty. content: %s", tmpMetaDir, files)
+		var found bool
+		for _, fi := range files {
+			if fi.Name() == ".trash" {
+				continue
+			}
+			found = true
+			break
+		}
+
+		if found {
+			t.Fatalf("%s: expected: empty, got: non-empty. content: %#v", tmpMetaDir, files)
 		}
 	}
 }