Replace consumer.accept by consumer.ack & consumer.nack in Pub/Sub #1864
Description
Attempting to tie together some offline discussion to get to a concrete resolution. The Pub/Sub ack ReplyConsumer [1] uses an enum for reply type. There seems to consensus that explicit .ack() and .nack() methods would be cleaner. Could we make this change before beta or disagree, @pongad @davidtorres @garrettjonesgoogle ?