Pub/Sub: Update default batch size to 10 MB (#4857)
Pub/Sub: Update default batch size to 10 MB (#4857)

The Pub/Sub default is 10 MB. This allows sending the max batch size unless the user intervenes.
jonparrott/gcloud-python,tseaver/gcloud-python,jonparrott/gcloud-python,dhermes/gcloud-python,tseaver/gcloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,GoogleCloudPlatform/gcloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,tseaver/google-cloud-python,dhermes/gcloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,googleapis/google-cloud-python,dhermes/google-cloud-python