Bug 667410 — A second aggregator instance only fetches a subset of contacts

This was happening because the initial BackendStore was hanging around across
multiple IndividualAggregator instances, keeping all the Backends,
PersonaStores and Personas alive.

The IndividualAggregator didn’t have code to deal with pre-prepared Backends
and PersonaStores, meaning it never realised the Personas existed (because
they weren’t announced via personas-changed signals), and thus never created
Individuals out of them.

This commit fixes the problem by having IndividualAggregator check for
existing Backends, PersonaStores and Personas when prepare() is called.

It also adds a test case to the folks test suite, based on the one written
by Guillaume in bgo#667410.

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=667410