Pull should set return_immediately to false

[pongad]

This came out of a discussion with Pubsub team.

cc @haakonringberg

[garrettjonesgoogle]

@pongad can you provide more context?

[pongad]

Slightly edited the description. IIUC, it saves RPC calls, but @haakonringberg can probably provide better details than I can.

[haakonringberg]

We now always discourage the use of return_immediately (it's an option we shouldn't have provided in the API). It's misleading (e.g., what does "immediately" mean in the context of a backlog?) and hurts throughput.