WIP
WIP
Remove Labellable

Error-message cost was too high for the flexibility. Sorry!

I actually like this better I think, since it doesn't introduce the
extra 'label' concept.
Mass renamings

The big change here is moving all the flow-builders under the `flow`
package, but there's a bunch of small changes and simplifications
elsewhere.
Exactly-once notes
Move notes to own directory
Remove some dead code from samza integration