Today, a setting can declare that its validity depends on the values of other
related settings. However, the validity of a setting is not always checked
against the correct values of its dependent settings because those settings'
correct values may not be available when the validator runs.
This commit separates the validation of a settings updates into two phases,
with separate methods on the `Setting.Validator` interface. In the first phase
the setting's validity is checked in isolation, and in the second phase it is
checked again against the values of its related settings. Most settings only
use the first phase, and only the few settings with dependencies make use of
the second phase.
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