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.

"  \"bestblock\":  \"hash\",    (string) The hash of the block at the tip of the chain\n"
            "  \"confirmations\" : n,       (numeric) The number of confirmations\n"
            "  \"value\" : x.xxx,           (numeric) The transaction value in " + CURRENCY_UNIT + "\n"
            "  \"scriptPubKey\" : {         (json object)\n"
            "     \"asm\" : \"code\",       (string) \n"
            "     \"hex\" : \"hex\",        (string) \n"
            "     \"reqSigs\" : n,          (numeric) Number of required signatures\n"
            "     \"type\" : \"pubkeyhash\", (string) The type, eg pubkeyhash\n"
            "     \"addresses\" : [          (array of string) array of bitcoin addresses\n"
            "        \"address\"     (string) bitcoin address\n"
            "        ,...\n"
            "     ]\n"
            "  },\n"
            "  \"coinbase\" : true|false   (boolean) Coinbase or not\n"
            "}\n"
                },
                RPCExamples{
            "\nGet unspent transactions\n"
            + HelpExampleCli("listunspent", "") +
            "\nView the details\n"
            + HelpExampleCli("gettxout", "\"txid\" 1") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("gettxout", "\"txid\", 1")
                },
            }.ToString());
    LOCK(cs_main);
    UniValue ret(UniValue::VOBJ);
    uint256 hash(ParseHashV(request.params[0], "txid"));
    int n = request.params[1].get_int();
    COutPoint out(hash, n);