orm: refactor orm.default_get() to avoid redundant computations

[xrg]

In default_get(), we need to evaluate the default value of each field,
in an order of resolution. The previous implementation did all the passes
regardless of the result of the previous step, and then dict.update()
the results.

Now, resolve from most-significant to least, trying only the remaining
fields at each step.
Also, have a provision for totally ignoring defaults from ir.values,
because they should not be considered when loading initial data.