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 (!state.IsValid()) {
        throw JSONRPCError(RPC_DATABASE_ERROR, FormatStateMessage(state));
    }
    return NullUniValue;
}
static UniValue reconsiderblock(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"reconsiderblock",
                "\nRemoves invalidity status of a block, its ancestors and its descendants, reconsider them for activation.\n"
                "This can be used to undo the effects of invalidateblock.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hash of the block to reconsider"},
                },
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("reconsiderblock", "\"blockhash\"")
            + HelpExampleRpc("reconsiderblock", "\"blockhash\"")
                },
            }.ToString());
    uint256 hash(ParseHashV(request.params[0], "blockhash"));
    {
        LOCK(cs_main);
        CBlockIndex* pblockindex = LookupBlockIndex(hash);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
        ResetBlockFailureFlags(pblockindex);
    }