resolved: clean up dns_transaction_stop()

resolved: clean up dns_transaction_stop()

This renames dns_transaction_stop() to dns_transaction_stop_timeout()
and makes it only about stopping the transaction timeout. This is safe,
as in most occasions we call dns_transaction_stop() at the same time as
dns_transaction_close_connection() anyway, which does the rest of what
dns_transaction_stop() used to do. And in the one where we don't call
it, it's implicitly called by the UDP emission or TCP connection code.

This also closes the connections as we enter the validation phase of a
transaction, so that no further messages may be received then.