async_receive2: race conditions on incomplete messages

[anarthal]

(Translating here a Slack conversation)

There is a potential race condition with async_receive2 as it is now:

• The server sends many pushes in a packet. Let's say 10, but the last one (the 10th) is incomplete. Some nodes for the 10th push are in the receive response, and some others aren't.
• Since there is at least one complete push, async_receive2 completes.
• User code ends up calling .clear()
• The final nodes for the 10th push are received. You now have an incomplete message delivered to the user

This is likely an indication that a plain generic_flat_response is too low-level to be used directly in PubSub.