When the mempool fills the verbose `getrawmempool` RPC call gets really slow.  It can take minutes for >100k transactions in the mempool.

The culprit is the univalue library.  Objects are implemented as a big list, but the library checks for duplicate keys.  So inserting n key-value pairs into an object takes O(n^2).

Possible fixes:

- Improve univalue library by using linked hash maps.
- change the RPC output format of getrawmempool to use a list of objects, instead of an object of objects.  But this breaks existing code.
- hack the library to be able to skip the duplicate check.

totalfee += txfee;
            
            CAmount feerate = weight ? (txfee * WITNESS_SCALE_FACTOR) / weight : 0;
            if (do_feerate_percentiles) {
                feerate_array.emplace_back(std::make_pair(feerate, weight));
            }
            maxfeerate = std::max(maxfeerate, feerate);
            minfeerate = std::min(minfeerate, feerate);
        }
    }
    CAmount feerate_percentiles[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
    CalculatePercentilesByWeight(feerate_percentiles, feerate_array, total_weight);
    UniValue feerates_res(UniValue::VARR);
    for (int64_t i = 0; i < NUM_GETBLOCKSTATS_PERCENTILES; i++) {
        feerates_res.push_back(feerate_percentiles[i]);
    }
    UniValue ret_all(UniValue::VOBJ);
    ret_all.pushKV("avgfee", (block.vtx.size() > 1) ? totalfee / (block.vtx.size() - 1) : 0);
    ret_all.pushKV("avgfeerate", total_weight ? (totalfee * WITNESS_SCALE_FACTOR) / total_weight : 0); 
    ret_all.pushKV("avgtxsize", (block.vtx.size() > 1) ? total_size / (block.vtx.size() - 1) : 0);
    ret_all.pushKV("blockhash", pindex->GetBlockHash().GetHex());
    ret_all.pushKV("feerate_percentiles", feerates_res);
    ret_all.pushKV("height", (int64_t)pindex->nHeight);
    ret_all.pushKV("ins", inputs);
    ret_all.pushKV("maxfee", maxfee);
    ret_all.pushKV("maxfeerate", maxfeerate);
    ret_all.pushKV("maxtxsize", maxtxsize);
    ret_all.pushKV("medianfee", CalculateTruncatedMedian(fee_array));
    ret_all.pushKV("mediantime", pindex->GetMedianTimePast());
    ret_all.pushKV("mediantxsize", CalculateTruncatedMedian(txsize_array));