Cancel all mempool download and verify tasks when a network upgrade activates

[teor2345]

Motivation

When a network upgrade activates, the mempool needs to redo validation with the new consensus rules.

Design

Tasks can await network upgrade activation using ChainTipChange::tip_change and TipAction::Reset:

zebra/zebra-state/src/service/chain_tip.rs

Line 313 in 44ac067

pub async fn tip_change(&mut self) -> Result<TipAction, watch::error::RecvError> {

[mpguerra]

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[mpguerra]

our approach will depend on what we do for #2374

[teor2345]

Currently Zebra:

• clears the mempool storage at network upgrade activation
• fetches transactions from peers using the mempool crawler
• validates those transactions using the new consensus rules using the mempool downloader

So I think the only thing that's missing here is clearing the download and verify queue.

The state send a Reset if there is a network upgrade activation.
In PR #2773, we cleared the storage on Reset, but not the download queue.

zebra/zebrad/src/components/mempool.rs

Lines 249 to 254 in 18acec6

	if let Some(tip_action) = self.chain_tip_change.last_tip_change() {
	match tip_action {
	// Clear the mempool if there has been a chain tip reset.
	TipAction::Reset { .. } => {
	storage.clear();
	}

[teor2345]

Edit: actually, it seems like we're missing clearing the download & verify stream