Use a UUID (rather than a sentinel object) on Pub / Sub `Future`. (#4634)
Use a UUID (rather than a sentinel object) on Pub / Sub `Future`. (#4634)

Also changing identity check to equality check for Future._SENTINEL.

GoogleCloudPlatform/gcloud-python,dhermes/google-cloud-python,tseaver/google-cloud-python,jonparrott/gcloud-python,googleapis/google-cloud-python,dhermes/gcloud-python,tseaver/gcloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,tseaver/gcloud-python,googleapis/google-cloud-python,dhermes/google-cloud-python,dhermes/google-cloud-python,jonparrott/gcloud-python,tseaver/google-cloud-python,GoogleCloudPlatform/gcloud-python,jonparrott/google-cloud-python,dhermes/gcloud-python,tswast/google-cloud-python,tswast/google-cloud-python