Adds a ready event to BackgroundConsumer to wait on start. (#7499)
Adds a ready event to BackgroundConsumer to wait on start. (#7499)

* Adds a ready event to BackgroundConsumer to allow waiting for the background thread to start


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