unbound: keep probing when servers are down

unbound: keep probing when servers are down

Till now when a server was in the "blocking regime" there was one probe
made every 15 min, to see if this server is up again. In situations
where all servers where down (e.g. because of a massive package loss)
it could take up to 15 min to have a working dns again.

This patch changes this behaviour in a way that a server marked down is
probed every 2 min.

Fixes: #12557
Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>