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{"getblockheader",
                "\nIf verbose is false, returns a string that is serialized, hex-encoded data for blockheader 'hash'.\n"
                "If verbose is true, returns an Object with information about blockheader <hash>.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The block hash"},
                    {"verbose", RPCArg::Type::BOOL,  "true", "true for a json object, false for the hex-encoded data"},
                },
                {
                    RPCResult{"for verbose = true",
            "{\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"
            "  \"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"