Auto-pause devices and pipelines

[gavv]

Requires these tasks to be completed first:

• #677
• #678
• #679
• #706

In sndio::Pump, we need to do the following:

• [ ] if source becomes idle:
  • [ ] if there is backup source, pause main source, switch to backup source, use it until main source becomes ready, then pause backup source, resume main source, and switch to it
  • [ ] if there is no backup source, pause source and sink, then sleep until source becomes ready, then resume source and sink
• [ ] if sink becomes idle:
  • [ ] pause source and sink, then sleep until sink becomes ready, then resume source and sink

After doing this, pump we will obtain the following behavior:

• on sender, while input device is idle, we don't send packets
• on receiver, while there are no sessions, we don't use device and CPU
• on receiver, while device is unavailable, we don't use device and CPU (requires #529)