This commit introduces a new AuthProvider and UserProvider that, similar to the
HybridAuthProvider and -UserProvider can be configured to use more than one backend
store. Where the Hybrids iterate over all of their providers in an attempt to
fulfill a request, these new 'Mapped' variants will first determine what provider
is to be used for a particular user, and will then operate on only that provider.
This is useful where particular users are required to be restricted to specific
backends.