tunnel-sink-new: Fix hang when used in combination with module-combine-sink

When tunnel-sink-new was used in combination with module-combine-sink, PA would hang because the main thread was blocked waiting for the execution of the latency snapshot message. The message would never be processed because the rtpoll associated with the control_inq of module-combine-sink was never run. This patch fixes the problem by running the rtpoll in the thread function to process incoming messages.

Fixes #1422 (closed)