When importing a watch-only address over `importmulti` with a specific timestamp, the wallet's `nTimeFirstKey` will be set to 1.
I think we should add a optional timestamp to `CWallet::AddWatchOnly()` and use it when importing addresses or pubkeys.

+ HelpExampleCli("createmultisig", "2 \"[\\\"16sSauSf5pF2UkUwvKGq4qjNRzBZYqgEL5\\\",\\\"171sgjn4YtPu27adkKGrdDwzRTxnRkBfKV\\\"]\"") +
            "\nAs a json rpc call\n"
            + HelpExampleRpc("createmultisig", "2, \"[\\\"16sSauSf5pF2UkUwvKGq4qjNRzBZYqgEL5\\\",\\\"171sgjn4YtPu27adkKGrdDwzRTxnRkBfKV\\\"]\"")
        ;
        throw runtime_error(msg);
    }
    
    CScript inner = _createmultisig_redeemScript(request.params);
    CScriptID innerID(inner);
    CBitcoinAddress address(innerID);
    UniValue result(UniValue::VOBJ);
    result.push_back(Pair("address", address.ToString()));
    result.push_back(Pair("redeemScript", HexStr(inner.begin(), inner.end())));
    return result;
}
UniValue verifymessage(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 3)
        throw runtime_error(
            "verifymessage \"address\" \"signature\" \"message\"\n"
            "\nVerify a signed message\n"
            "\nArguments:\n"
            "1. \"address\"         (string, required) The bitcoin address to use for the signature.\n"
            "2. \"signature\"       (string, required) The signature provided by the signer in base 64 encoding (see signmessage).\n"
            "3. \"message\"         (string, required) The message that was signed.\n"
            "\nResult:\n"
            "true|false   (boolean) If the signature is verified or not.\n"
            "\nExamples:\n"
            "\nUnlock the wallet for 30 seconds\n"
            + HelpExampleCli("walletpassphrase", "\"mypassphrase\" 30") +