Allowing calls to select_connect_backend_servers even when all slaves are
connected solves the debug assertion in select_connect_backend_servers
that happens when the execution of a queued query causes a new connection
to be created.
Auto types aren't very neat as return values because they move the burden
of knowledge to the reader. Using an explicit, and somewhat
self-explanatory, type makes it easier to assess the code without knowing
the implementation of the type.
