The output contains an op ID, to make it easier to distinguish parallel
operations.

For #8.

If these ever prove to be desirable, we can revert this or add a mount
option.

While I'm thinking about GoogleCloudPlatform/gcsfuse#23.

For GoogleCloudPlatform/gcsfuse#30.

To my great disappointment, I can't figure out a way to make this test
reliable on Linux. Hopefully the cited mailing list thread will come
through.

For #7.