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() < 1 || request.params.size() > 2)
        throw std::runtime_error(
            RPCHelpMan{"getblock",
                "\nIf verbosity is 0, returns a string that is serialized, hex-encoded data for block 'hash'.\n"
                "If verbosity is 1, returns an Object with information about block <hash>.\n"
                "If verbosity is 2, returns an Object with information about block <hash> and information about each transaction. \n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The block hash"},
                    {"verbosity", RPCArg::Type::NUM,  "1", "0 for hex-encoded data, 1 for a json object, and 2 for json object with transaction data"},
                },
                {
                    RPCResult{"for verbosity = 0",
            "\"data\"             (string) A string that is serialized, hex-encoded data for block 'hash'.\n"
                    },
                    RPCResult{"for verbosity = 1",
            "{\n"
            "  \"hash\" : \"hash\",     (string) the block hash (same as provided)\n"
            "  \"confirmations\" : n,   (numeric) The number of confirmations, or -1 if the block is not on the main chain\n"
            "  \"size\" : n,            (numeric) The block size\n"
            "  \"strippedsize\" : n,    (numeric) The block size excluding witness data\n"
            "  \"weight\" : n           (numeric) The block weight as defined in BIP 141\n"
            "  \"height\" : n,          (numeric) The block height or index\n"
            "  \"version\" : n,         (numeric) The block version\n"
            "  \"versionHex\" : \"00000000\", (string) The block version formatted in hexadecimal\n"
            "  \"merkleroot\" : \"xxxx\", (string) The merkle root\n"
            "  \"tx\" : [               (array of string) The transaction ids\n"
            "     \"transactionid\"     (string) The transaction id\n"
            "     ,...\n"
            "  ],\n"