Build a common failover (fo_ctx) backend infrastructure used
by all providers where providers can register their own servers and
services (ipa will use one common service so all providers use the same
server at all time).
The common infrastructure will provide a way to register callbacks so
that each provider is notified if any of the other marks a server as
bad, and a new server is selected. So that they can do whatever is
necessary to immediately switch to the new server w/o having to find out
it is bad themselves.