feat(rpc)!: Update Emitter::mempool to support evicted_at

[LagginTimes]

Description

Work for this began as part of #1811, based on this comment.

Emitter::mempool now returns a MempoolEvent which provides the data for tracking evicted_at:

pub struct MempoolEvent {
    /// Unemitted transactions or transactions with ancestors that are unseen by the receiver.
    ///
    /// To understand the second condition, consider a receiver which filters transactions based on
    /// whether it alters the UTXO set of tracked script pubkeys. If an emitted mempool transaction
    /// spends a tracked UTXO which is confirmed at height `h`, but the receiver has only seen up to
    /// block of height `h-1`, we want to re-emit this transaction until the receiver has seen the
    /// block at height `h`.
    pub new_txs: Vec<(Transaction, u64)>,

    /// [`Txid`]s of all transactions currently present in the mempool.
    ///
    /// This is useful to determine whether transactions are evicted from the mempool.
    pub all_txids: HashSet<Txid>,

    /// The latest timestamp of when a transaction entered the mempool.
    ///
    /// This is useful for setting the timestamp for evicted transactions.
    pub latest_update_time: u64,
}

Changelog notice

• Emitter::mempool now returns a MempoolEvent.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[evanlinjin]

Instead of rebasing, it's probably best to git switch -c <new-branch> from #1839 and cherry-pick on the new commits.