Improve async client performance

I threw together a quick benchmark, and was seeing ~4-5x slower async calls vs sync (with the network mocked out)

This mostly appears to be due to re-building the wrapped function on each call, rather than caching. I opened googleapis/gapic-generator-python#1755 to address this

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.