Unjostable queries for configured upstreams

[gthess]

Draft the idea of unjostable queries if upstreams are explicitly configured. May prove useful if Unbound's general upstream connectivity is problematic and request list keeps growing. Then a client query for a configured stub-zone for example would not be eligible to be replaced but instead allowed to run to completion.

That would need explicit configuration and not what this PR is initially doing, i.e., treating every configured upstream as non-eligible for replacement. A better configuration would be to define a custom (higher) jostle-timeout for those queries to persist for a while longer before being replaced.