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.

int nVersion = s.GetVersion();
        if (!(s.GetType() & SER_GETHASH))
            READWRITE(nVersion);
        
        READWRITE(nCreditDebit);
        READWRITE(nTime);
        READWRITE(LIMITED_STRING(strOtherAccount, 65536));
        if (!ser_action.ForRead())
        {
            WriteOrderPos(nOrderPos, mapValue);
            if (!(mapValue.empty() && _ssExtra.empty()))
            {
                CDataStream ss(s.GetType(), s.GetVersion());
                ss.insert(ss.begin(), '\0');
                ss << mapValue;
                ss.insert(ss.end(), _ssExtra.begin(), _ssExtra.end());
                strComment.append(ss.str());
            }
        }
        READWRITE(LIMITED_STRING(strComment, 65536));
        size_t nSepPos = strComment.find("\0", 0, 1);
        if (ser_action.ForRead())
        {
            mapValue.clear();
            if (std::string::npos != nSepPos)
            {
                CDataStream ss(std::vector<char>(strComment.begin() + nSepPos + 1, strComment.end()), s.GetType(), s.GetVersion());
                ss >> mapValue;
                _ssExtra = std::vector<char>(ss.begin(), ss.end());