s3: implement --s3-upload-cutoff for single part uploads below this - fixes #2772

Before this change rclone would use multipart uploads for any size of
file.  However multipart uploads are less efficient for smaller files
and don't have MD5 checksums so it is advantageous to use single part
uploads if possible.

This implements single part uploads for all files smaller than the
upload_cutoff size.  Streamed files must be uploaded as multipart
files though.

backend/s3/s3_test.go

@@ -23,4 +23,8 @@ func (f *Fs) SetUploadChunkSize(cs fs.SizeSuffix) (fs.SizeSuffix, error) {
 	return f.setUploadChunkSize(cs)
 }
 
+func (f *Fs) SetUploadCutoff(cs fs.SizeSuffix) (fs.SizeSuffix, error) {
+	return f.setUploadCutoff(cs)
+}
+
 var _ fstests.SetUploadChunkSizer = (*Fs)(nil)