Refactor stale lookups, ignore active RRsets

Refactor stale lookups, ignore active RRsets

When doing a staleonly lookup, ignore active RRsets from cache. If we
don't, we may add a duplicate RRset to the message, and hit an
assertion failure in query.c because adding the duplicate RRset to the
ANSWER section failed.

This can happen on a race condition. When a client query is received,
the recursion is started. When 'stale-answer-client-timeout' triggers
around the same time the recursion completes, the following sequence
of events may happen:
1. Queue the "try stale" fetch_callback() event to the client task.
2. Add the RRsets from the authoritative response to the cache.
3. Queue the "fetch complete" fetch_callback() event to the client task.
4. Execute the "try stale" fetch_callback(), which retrieves the
   just-inserted RRset from the database.
5. In "ns_query_done()" we are still recursing, but the "staleonly"
   query attribute has already been cleared. In other words, the
   query will resume when recursion ends (it already has ended but is
   still on the task queue).
6. Execute the "fetch complete" fetch_callback(). It finds the answer
   from recursion in the cache again and tries to add the duplicate to
   the answer section.

This commit changes the logic for finding stale answers in the cache,
such that on "stale_only" lookups actually only stale RRsets are
considered. It refactors the code so that code paths for "dbfind_stale",
"stale_refresh_window", and "stale_only" are more clear.

First we call some generic code that applies in all three cases,
formatting the domain name for logging purposes, increment the
trystale stats, and check if we actually found stale data that we can
use.

The "dbfind_stale" lookup will return SERVFAIL if we didn't found a
usable answer, otherwise we will continue with the lookup
(query_gotanswer()). This is no different as before the introduction of
"stale-answer-client-timeout" and "stale-refresh-time".

The "stale_refresh_window" lookup is similar to the "dbfind_stale"
lookup: return SERVFAIL if we didn't found a usable answer, otherwise
continue with the lookup (query_gotanswer()).

Finally the "stale_only" lookup.

If the "stale_only" lookup was triggered because of an actual client
timeout (stale-answer-client-timeout > 0), and if database lookup
returned a stale usable RRset, trigger a response to the client.
Otherwise return and wait until the recursion completes (or the
resolver query times out).

If the "stale_only" lookup is a "stale-anwer-client-timeout 0" lookup,
preferring stale data over a lookup. In this case if there was no stale
data, or the data was not a positive answer, retry the lookup with the
stale options cleared, a.k.a. a normal lookup. Otherwise, continue
with the lookup (query_gotanswer()) and refresh the stale RRset. This
will trigger a response to the client, but will not detach the handle
because a fetch will be created to refresh the RRset.