Have IO serialization be opt-in/-out

[liamhuber]

Joerg requested that we give node designers explicit access to a flag specifying the priority to serialize port values on session serialization. By default I imagine all input values should have a high serialization priority, while all output values should by default have a low serialization priority. Values could be on a scalar field, and the user could then specify a priority threshold for serialization.

I can imagine also exposing this priority in the node controller, so it can be tweaked for particular graphs if you really want to optimize storage -- e.g. even DataNodes don't need to bother serializing their input if it's already connected.

For that matter, maybe we can automate it so that being connected automatically drops the serialization priority for input...