Pipeline pause/resume

[gavv]

• [ ] Sender: pause stops session, resume restarts session. While paused, sender drops all frames and reports paused state.
• [ ] Receiver: pause stops all sessions, resume restarts them. While paused, receiver does not fetch packets, produces zeros, and reports paused state.
• [ ] Add roc_sender_pause()/roc_sender_resume() and roc_receiver_pause()/roc_receiver_resume() to C API.
• [ ] Add pipeline tests.
• [ ] Add public_api tests.

Should be done after #677.