Performance of onChange & event validation

There is a performance cost to forcing a re-render of the form's containing component when something changes.

Figure out and document some guidelines for writing your own `BoundField`-rendering React component to implement `shouldComponentUpdate()`.

Other things which will help in the short term:
- Providing a way to get all a field's mutable state from a `BoundField`
- Use `React.addons.update()` for state changes

Longer term:
- Use an immutable library for state objects.
- Switch to rendering React components for field rows & widgets

Start by tracing how BoundField-rendering React components trigger containing-form re-renders and how their mutable state is exposed. Review shouldComponentUpdate(), React.addons.update(), and the listed short- and long-term ideas. Done means documented guidance for custom BoundField components plus an agreed scope for the proposed performance changes.

Written by the indexing model from the issue text.