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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* filter using minimum-sized, 60byte, transactions might manage to send
     * 1000/sec if we have fast peers, so we pick 120,000 to give our peers a
     * two minute window to send invs to us.
     *
     * Decreasing the false positive rate is fairly cheap, so we pick one in a
     * million to make it highly unlikely for users to have issues with this
     * filter.
     *
     * We typically only add wtxids to this filter. For non-segwit
     * transactions, the txid == wtxid, so this only prevents us from
     * re-downloading non-segwit transactions when communicating with
     * non-wtxidrelay peers -- which is important for avoiding malleation
     * attacks that could otherwise interfere with transaction relay from
     * non-wtxidrelay peers. For communicating with wtxidrelay peers, having
     * the reject filter store wtxids is exactly what we want to avoid
     * redownload of a rejected transaction.
     *
     * In cases where we can tell that a segwit transaction will fail
     * validation no matter the witness, we may add the txid of such
     * transaction to the filter as well. This can be helpful when
     * communicating with txid-relay peers or if we were to otherwise fetch a
     * transaction via txid (eg in our orphan handling).
     *
     * Memory used: 1.3 MB
     */
    std::unique_ptr<CRollingBloomFilter> recentRejects GUARDED_BY(cs_main);
    uint256 hashRecentRejectsChainTip GUARDED_BY(cs_main);
    /*
     * Filter for transactions that have been recently confirmed.
     * We use this to avoid requesting transactions that have already been