A few thoughts on unifying RRR

All,

During our last call we discussed how we could unify rebuild, recalculate,
and revalidate (RRR). The thinking was that currently the spec is a little
too prescriptive.

For example, there is no clear use case for having the ability to run
`recalculate` and `revalidate` independently, yet these operations have
their own actions and events. I have found that whenever you want to
recalculate, you almost always want a revalidate. Similarly, when you want
a rebuild, you almost always want a recalculate and a revalidate. So you
end up writing:

    <action>
      <rebuild/>
      <recalculate/>
      <revalidate/>
    </action>

If we were to pursue this further, I think we should have a single "update
model" action. In fact it could be even called simply `update`:

    <update/>

That action would target a single model. Once it completes, the expected
result would be that:

- the dependency graph is up to date
- calculations and MIPs are up to date
- validations are up to date

The action should be "fast". It can still use the deferred update flags
which XForms specifies (rebuild, recalculate, revalidate flags). It can
also use other optimizations as it sees fit.

In the case of dynamic XPath dependencies, if we cannot mandate that an
implementation detects every possible such dependencies, the `update`
action could have a `force` flag:

    <update force="true"/>

This would force the processor to rebuild everything.

The concepts of rebuild, recalculate, and revalidate still make sense. It's
just that they are parts of a larger encompassing model update process.

-Erik