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.

UniValue ret(UniValue::VOBJ);
    CCoinsStats stats;
    FlushStateToDisk();
    if (GetUTXOStats(pcoinsdbview.get(), stats)) {
        ret.pushKV("height", (int64_t)stats.nHeight);
        ret.pushKV("bestblock", stats.hashBlock.GetHex());
        ret.pushKV("transactions", (int64_t)stats.nTransactions);
        ret.pushKV("txouts", (int64_t)stats.nTransactionOutputs);
        ret.pushKV("bogosize", (int64_t)stats.nBogoSize);
        ret.pushKV("hash_serialized_2", stats.hashSerialized.GetHex());
        ret.pushKV("disk_size", stats.nDiskSize);
        ret.pushKV("total_amount", ValueFromAmount(stats.nTotalAmount));
    } else {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Unable to read UTXO set");
    }
    return ret;
}
UniValue gettxout(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() < 2 || request.params.size() > 3)
        throw std::runtime_error(
            RPCHelpMan{"gettxout",
                "\nReturns details about an unspent transaction output.\n",
                {
                    {"txid", RPCArg::Type::STR, RPCArg::Optional::NO, "The transaction id"},
                    {"n", RPCArg::Type::NUM, RPCArg::Optional::NO, "vout number"},
                    {"include_mempool", RPCArg::Type::BOOL,  "true", "Whether to include the mempool. Note that an unspent output that is spent in the mempool won't appear."},
                },
                RPCResult{
            "{\n"