Broadcast AdvertiseBlock to peers when we commit a new block

[teor2345]

Motivation

Zebra needs to gossip the hashes of newly committed blocks, once it has reached the chain tip.

Design

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> {

Both Grow and Reset variants should result in block gossips.

[mpguerra]

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