core: Ensure we always notify of new Individuals

In the following situation, it was possible for
IndividualAggregator.individuals_changed_detailed to not emit a notification
that an individual was added:

If two personas were added by a given store in the same emission of
PersonaStore.personas_changed, the IA would create an Individual, i1, from
the first and add a mapping (null → i1) to the change set. It would then
process the second, destroying the first individual and creating a new
Individual, i2, (correctly) containing both personas. In doing so, it would
remove the mapping (null → i1) from the change set, but would incorrectly
not add a mapping (null → i2) in its place.

This situation can be extended to others where a single new Individual is
formed from multiple new personas coming from a single emission of
PersonaStore.personas_changed.

Closes: bgo#657282 (again)