Pipeline state polling

[gavv]

• [ ] Add pipeline::StateTracker::wait_state() that allows to block (from any thread) until state is changed to one in the specified mask. It should support timeout argument (with an option for no timeout and infinite timeout).
• [ ] Add sndio::IDevice::has_poll() and sndio::IDevice::poll() and implement it in ReceiverSource and SenderSink using StateTracker.
• [ ] Add roc_receiver_poll() and roc_sender_poll() to C API.
• [ ] Add pipeline tests.
• [ ] Add public_api tests.

Needed for #473 abd #680.