net: factor urgency into transaction selection priority #35110

Remove the snapshot-only wrapper types and let TxBroadcastInfo serve
as both the stored private broadcast state and the snapshot returned
by GetBroadcastInfo().

This keeps the cleanup narrow, preserves the getprivatebroadcastinfo
JSON output, and does not change behavior.

As a narrow tradeoff, the m_transactions value now stores
TxBroadcastInfo, which includes tx, so CTransactionRef is present in
both the map key and value. Keeping that duplication removes the extra
wrapper type without pulling a broader structural change into this
cleanup.

Move the private_broadcast_tests mock-time handling to
NodeClockContext.

Keeping this test-only time-handling cleanup separate makes the
private broadcast state/snapshot cleanup easier to review.

Previously, PickTxForSend() would always prioritize transactions with
fewer send attempts, even if those transactions had been waiting much
longer than newer alternatives. This could result in long-waiting
transactions being perpetually deprioritized.

The priority function now includes an urgency factor based on elapsed
time since the first send attempt. Urgency is placed after the
existing fairness metrics (num_picked, num_confirmed) so that it only
breaks ties between transactions with equal send history. This ensures
that transactions which have been waiting significantly longer will
eventually be selected over fresh alternatives.

No behavior change for transactions with different send attempt counts.