Pub/Sub: Mitigate message duplication to due stuck stream-close signal

[kir-titievsky]

As documented offline, Pub/Sub streamingPull may result in a high duplicate rate because the server terminates a streamingPull connection by sending a signal down the stream. This signal is delivered to the client after the messages already in the stream buffer are processed. As a result, the client may send acks and modifyAckDeadline's up a stream that has already been broken, leading to their loss.

To close this bug:

• Agree on a mitigation strategy. Currently, it is to send acks and modifyAckDeadline's as separate requests (synchronous or streaming?)
• Implement in Java.
• Validate that the implementation does not lead to performance degradation or extreme delays in ack and modAck delivery to the server, which would again lead to high rate of duplication.
• Reproduce in other clients.

This is GA launch blocking.

[pongad]

The reference implementation in Java is already completed in #3123

[pongad]

From meeting today, this was closed by #3123