Start disconnecting from outdated peers a few days before network upgrade activation

[teor2345]

Motivation

To avoid Zcash peer connection storms, we want to start disconnecting from outdated peers before network upgrade activation.

Specifications

Here is what zcashd does:
https://zips.z.cash/zip-0201#network-coalescence

Designs

1. If Zebra is close to a network upgrade, at regular intervals, randomly choose an outdated peer, and disconnect from it. This will make Zebra automatically reconnect to another peer in its address book.
2. Zebra attempts new peer connections in a fixed, predictable order #1875 might be needed to make sure we don't just reconnect to the same peer
3. We might be able to re-use the ChangePeerConnections request from Tracking: security: Limit ability of synthetic nodes to take up connection slots. Credit: Ziggurat Team  #7822, but add an "outdated peers only" filter to it.

Possible Implementation

Launch a peer set background task that waits until the block height has changed, checks if it is near a network upgrade, then sends a ChangePeerConnections { disconnect_outdated: true } request.

Then sleep until the next request could be sent, whether a request was sent or not. This avoids locking the state watch channel too much.

Out of Scope

optional changes

5. When connecting to peers, randomly choose a peer from reconnection_peers, rather than always choosing the first one. To prefer peers that are more likely to reconnect, randomly select two peer indexes using sample, then return the peer at the lower index.

Compatibility

Here are the zcashd parameters:

• close means height >= (activation_height - NETWORK_UPGRADE_PEER_PREFERENCE_BLOCK_PERIOD)
• NETWORK_UPGRADE_PEER_PREFERENCE_BLOCK_PERIOD is 1728, or 1.5 days
• the regular intervals are whenever there is a new connection
• an outdated peer is a peer that would be disconnected after the upcoming network upgrade

For Zebra, we could use:

• a regular interval of 5 minutes, but only if we have more than 2 peers, and
• the same parameters as zcashd

This makes the number of disconnections 2-6 times the peer set size. (The peer limits are 75 outbound and 125 inbound peers.) So if half the peers on the network are upgraded, Zebra should be connected to 75% - 98% upgraded peers when the network upgrade activates.

[mpguerra]

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