Increment BQ DB-API thread safety. (#3693)
Increment BQ DB-API thread safety. (#3693)

Increment to 2 per https://www.python.org/dev/peps/pep-0249/#threadsafety. The cursor object includes some state for paging through results and other things which are not protected by locs.

Closes #3522.
googleapis/google-cloud-python,GoogleCloudPlatform/gcloud-python,googleapis/google-cloud-python,calpeyser/google-cloud-python,calpeyser/google-cloud-python,tseaver/google-cloud-python,tseaver/gcloud-python,jonparrott/gcloud-python,dhermes/google-cloud-python,tartavull/google-cloud-python,jonparrott/google-cloud-python,tartavull/google-cloud-python,dhermes/google-cloud-python,dhermes/gcloud-python,tswast/google-cloud-python,tseaver/gcloud-python,tseaver/google-cloud-python,tseaver/google-cloud-python,jonparrott/gcloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,dhermes/gcloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,tswast/google-cloud-python