Fix a serve-stale issue with a delegated zone

Fix a serve-stale issue with a delegated zone

When 'stale-answer-client-timeout' is 0, named is allowed to return
a stale answer immediately, while also initiating a new query to get
the real answer. This mode is activated in ns__query_start() by setting
the 'qctx->options.stalefirst' optoin to 'true' before calling the
query_lookup() function, but not when the zone is known to be
authoritative to the server. When the zone is authoritative, and
query_looup() finds out that the requested name is a delegation,
then before proceeding with the query, named tries to look it up
in the cache first. Here comes the issue that it doesn't consider
enabling 'qctx->options.stalefirst' in this case, and so the
'stale-answer-client-timeout 0' setting doesn't work for those
delegated zones - instead of immediately returning the stale answer
(if it exists), named tries to resolve it.

Fix this issue by enabling 'qctx->options.stalefirst' in the
query_zone_delegation() function just before named looks up the name
in the cache using a new query_lookup() call. Also, if nothing was
found in the cache, don't initiate another query_lookup() from inside
query_notfound(), and let query_notfound() do its work, i.e. it will
call query_delegation() for further processing.