cp command hangs when uploading large files

```
/usr/local/bin/gsutil -o GSUtil:parallel_composite_upload_threshold=10M -D cp gs://google-bucket/myfile.gz s3://s3-bucket/s3dir/
```

```
$ gsutil version -l
gsutil version: 4.26
checksum: PACKAGED_GSUTIL_INSTALLS_DO_NOT_HAVE_CHECKSUMS (!= 7dec1285a94a82cf663408e343b2dd66)
boto version: 2.46.1
python version: 2.7.12
OS: Linux xxx.amzn1.x86_64
multiprocessing available: True
using cloud sdk: False
config path(s): /home/myhome/.boto
gsutil path: /usr/local/bin/gsutil
compiled crcmod: True
installed via package manager: True
editable install: False
```