There are some potential feefilter cleanups mentioned and motivated in comment https://github.com/bitcoin/bitcoin/pull/21509#issuecomment-805808485 . If you address the locking one, please be careful and make sure that `round()` is only called under a lock. See https://github.com/bitcoin/bitcoin/pull/21509#issuecomment-807992199

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/**
     * Map of all Peer objects, keyed by peer id. This map is protected
     * by the m_peer_mutex. Once a shared pointer reference is
     * taken, the lock may be released. Individual fields are protected by
     * their own locks.
     */
    std::map<NodeId, PeerRef> m_peer_map GUARDED_BY(m_peer_mutex);
    
    int nSyncStarted GUARDED_BY(cs_main) = 0;
    /**
     * Sources of received blocks, saved to be able punish them when processing
     * happens afterwards.
     * Set mapBlockSource[hash].second to false if the node should not be
     * punished if the block is invalid.
     */
    std::map<uint256, std::pair<NodeId, bool>> mapBlockSource GUARDED_BY(cs_main);
    
    int m_wtxid_relay_peers GUARDED_BY(cs_main) = 0;
    
    int m_outbound_peers_with_protect_from_disconnect GUARDED_BY(cs_main) = 0;
    bool AlreadyHaveTx(const GenTxid& gtxid) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    /**
     * Filter for transactions that were recently rejected by
     * AcceptToMemoryPool. These are not rerequested until the chain tip
     * changes, at which point the entire filter is reset.
     *
     * Without this filter we'd be re-requesting txs from each of our peers,
     * increasing bandwidth consumption considerably. For instance, with 100
     * peers, half of which relay a tx we don't accept, that might be a 50x
     * bandwidth increase. A flooding attacker attempting to roll-over the