resolved: use reference counting for DnsQueryCandidate objects

resolved: use reference counting for DnsQueryCandidate objects

Follow-up for 4ea8b443de. The logic that manages DnsQueryCandidate is rather
complicated: a calls to dns_query_complete() that wants to destroy a
DnsQueryCandidate can be nested inside a deep chain of calls. Using reference
counts seems like the simplest approach.

DnsSearchDomain already uses reference counting.

This patch effectively brings dns_query_candidate_go() to the state before
4ea8b443de, but wraps the iteration over DnsQueryCandidate.transactions in
dns_query_candidate_ref+dns_query_candidate_unref.