resolved: introduce dns_transaction_key() helper for getting RR key for transaction

resolved: introduce dns_transaction_key() helper for getting RR key for transaction

This is a simple search&replace excercise: instead of accessing the
"key" field of the transaction directly, let's use a small inline helper
that does this for us.

This appears pointless for now, but this will become useful later when
we introduce "bypass" transactions, that reuse the original client DNS
packet for queries instead of synthesizing a packet of our own. In that
case transactions either have regular "key" field initialized as before,
or the "bypass" packet field instead. The new dns_transaction_key()
helper allows us to hide the differences for most cases as we can later
teach it to access the "bypass" packet's question key transparently.

No change in behaviour.