sink, source: Add a latency offset which is inherited from the port

sink, source: Add a latency offset which is inherited from the port

A latency offset variable was added to the sink/source struct.

Also a function was introduced to update the latency offset of the
sink/source and a new message type was introduced so we can send the latency
offset to the IO thread.

The latency offset is automatically populated with the latency from the
currently active port.