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.

{
    if (request.fHelp || request.params.size() != 0)
        throw std::runtime_error(
            RPCHelpMan{"getmempoolinfo",
                "\nReturns details on the active state of the TX memory pool.\n",
                {},
                RPCResult{
            "{\n"
            "  \"size\": xxxxx,               (numeric) Current tx count\n"
            "  \"bytes\": xxxxx,              (numeric) Sum of all virtual transaction sizes as defined in BIP 141. Differs from actual serialized size because witness data is discounted\n"
            "  \"usage\": xxxxx,              (numeric) Total memory usage for the mempool\n"
            "  \"maxmempool\": xxxxx,         (numeric) Maximum memory usage for the mempool\n"
            "  \"mempoolminfee\": xxxxx       (numeric) Minimum fee rate in " + CURRENCY_UNIT + "/kB for tx to be accepted. Is the maximum of minrelaytxfee and minimum mempool fee\n"
            "  \"minrelaytxfee\": xxxxx       (numeric) Current minimum relay fee for transactions\n"
            "}\n"
                },
                RPCExamples{
                    HelpExampleCli("getmempoolinfo", "")
            + HelpExampleRpc("getmempoolinfo", "")
                },
            }.ToString());
    return MempoolInfoToJSON(::mempool);
}
static UniValue preciousblock(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(