resolved: lower SERVFAIL cache timeout from 30s to 10s

Apparently 30s is a bit too long for some cases, see #5552. But not
caching SERVFAIL at all also breaks stuff, see explanation in
201d9958.

Let's try to find some middle ground, by lowering the cache timeout to
10s. This should be ample for the problem
201d9958 attackes, but not as long as
half a miute, as #5552 complains.

Fixes: #5552
(cherry picked from commit 19bcef9d)