resolved: block transaction GC'ing while dns_transaction_request_dnssec_keys() is...

resolved: block transaction GC'ing while dns_transaction_request_dnssec_keys() is running

If any of the transactions started by
dns_transaction_request_dnssec_keys() finishes promptly without
requiring asynchronous operation this is reported back to the issuing
transaction from the same stackframe. This might ultimately result in
this transaction to be freed while we are still in its
_request_dnssec_keys() stack frame. To avoid memory corruption block the
transaction GC while in the call, and manually issue a GC after it
returned.