Allows to hook into the config loading process just before OnAllConfigLoaded()
is called on a bunch of individual config objects. Allows doing some operations
more efficiently at once for all objects.
Intended use: when adding a number of dependencies, it has to be checked
whether this uses any cycles. This can be done more efficiently if all
dependencies are checked at once. So far, this is with a case-distinction for
initially loaded files in DaemonUtility::LoadConfigFiles() and for dependencies
created by runtime updates in Dependency::OnAllConfigLoaded(). The mechanism
added by this commit allows to unify the handling of both cases (done in a
following commit).
