s3: set upload concurrency to 2

we upload a file while receiving it via SFTP not a file stored on a local
disk. We use concurrent uploads only to be able to send files of arbitrary
size, so concurrency is not really useful here. Setting the concurrency to
2 we have a max difference of 10 MB between the writer (sftp client) and
the reader (aws sdk), with the default concurrency value this difference
is 25MB.

vfs/s3fs.go

@@ -188,6 +188,8 @@ func (fs S3Fs) Create(name string, flag int) (*os.File, *pipeat.PipeWriterAt, fu
 			Key:          aws.String(key),
 			Body:         r,
 			StorageClass: utils.NilIfEmpty(fs.config.StorageClass),
+		}, func(u *s3manager.Uploader) {
+			u.Concurrency = 2
 		})
 		r.CloseWithError(err)
 		fsLog(fs, logger.LevelDebug, "upload completed, path: %#v, response: %v, readed bytes: %v, err: %v",