Serve expired cache update fixes

[gthess]

This tries to fix certain things related to serve-expired and cache updates:

• If resolution fails, back off and use the expired entry for some time (default 5 seconds)
• If serve-expired is set (but not serve-expired-client-timeout) still try to replace a SERVFAIL answer with an expired reponse
• Exclude explicit RRSIG queries to require validation before being cached

[wcawijngaards]

Comments elsewhere, but approved. Important to have a limit on work for expired contents that is served, otherwise every incoming query could turn into an upstream request, a request attempt for failing lookups.

[wcawijngaards]

The changes look good! With serve expired, the norec_ttl is adjusted to stop repeated attempts, for a short duration, and the additional fix is a catch for additional cases where this is needed.