Gossip recently verified block hashes to peers

[teor2345]

Motivation

Zebra needs to broadcast verified block hashes to its peers, so they learn about new blocks.

Designs

Grow and Reset chain tip changes should be sent in block gossips.

Tasks can wait until they are near the tip using SyncStatus::wait_until_close_to_tip:

zebra/zebrad/src/components/sync/status.rs

Lines 31 to 34 in 44ac067

	/// Wait until the synchronization is likely close to the tip.
	///
	/// Returns an error if communication with the synchronizer is lost.
	pub async fn wait_until_close_to_tip(&mut self) -> Result<(), watch::error::RecvError> {

Tasks can await tip changes using ChainTipChange::tip_change and TipAction:

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

Line 313 in 44ac067

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

Solution

• wait for state best tip changes
• wait until Zebra is close to the tip
• make sure we have the latest tip change
• broadcast each change to all ready peers
• test that committed blocks are gossiped to peers

Closes #2712.

Testing

Here are the manual tests I did:

• run on an ephemeral instance: no gossips during initial sync
• run on a synced instance: gossips after the 100 non-finalized blocks are synced
• looked at the outbound inv messages using the Grafana dashboard: more inv messages than before this change

Review

@jvff or @oxarbitrage can review this PR.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[jvff]

Looking good so far. I added one idea about how to try to simplify the code.

[teor2345]

I manually tested this PR with:

• fully synced and new ephemeral nodes
• on testnet and mainnet

And it works without panicking (so #2789 and #2800 have not regressed).

[teor2345]

(This is lower priority, because it's not blocking any other work.)

[oxarbitrage]

Looks great to me, i made just a few very minor comments. Feel free to merge here.