Make an object for a processing node.
Implement a sink node. Make it possible to implement Sink and Source
interfaces to provide input/output from the node.
Improve pinosdepay to track fds and handle format changes.
Rename the source-output object to channel because it is used for both
input and output.
Start the beginnings of sink support. This will make it possible to make
pinos consume data as well as provide data.
Detect a format change in the payloader and post an element message.
Catch the format change element message in the client-source and use it
to update the format and possible formats in the source and source-outputs.
