fix(pubsub): Switch to a last-seen cache strategy

[Stebalien]

Proposed Changes

We were using the default first-seen strategy which means we'd loop messages every 2 minutes (when they expire from the cache). This new strategy keeps the message in the cache until we haven't seen it for 2 minutes, ensuring that it eventually drops off the network (with high likelihood).

The downside to this change, and a reason to potentially not include it, is that it may significantly harm message propegation. Specifically:

1. Node sends a message.
2. Node's peers propegate it.
3. Peer re-sends it.
4. Nodes peers (the same peers) have already seen the message so they don't propagate it. But some other nodes on the network didn't get it, so the message is censored.

In practice, we've configured our re-broadcast interval to be greater than the cache time so this may not be an issue? But this still isn't a straight-forward merge.

Additional Info

The correct fix is message expiration: libp2p/go-libp2p-pubsub#573, but that requires deeper protocol changes.

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title conforms with contribution conventions
• Update CHANGELOG.md or signal that this change does not need it per contribution conventions
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[github-actions]

Please update the PR title to match https://github.com/filecoin-project/lotus/blob/master/CONTRIBUTING.md#pr-title-conventions