Make a new DRAINED status.
Place the DRAINED status on an input IO when a stream is out of
buffers and draining.
All nodes that don't have HAVE_DATA on the input io need to copy
it to the output io and return the status. This makes sure the
DRAINED is forwarded and nodes return DRAINED from _process()
DRAINED on the resampler flushes out the last queued samples and then
forwards the DRAINED in the next iteration.
Emit a new drained signal from the context when a node returns
DRAINED. Use this to trigger the drained signal in the stream.