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.

ifstream file;
    file.open(request.params[0].get_str().c_str(), std::ios::in | std::ios::ate);
    if (!file.is_open())
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot open wallet dump file");
    int64_t nTimeBegin = chainActive.Tip()->GetBlockTime();
    bool fGood = true;
    int64_t nFilesize = std::max((int64_t)1, (int64_t)file.tellg());
    file.seekg(0, file.beg);
    pwalletMain->ShowProgress(_("Importing..."), 0); 
    while (file.good()) {
        pwalletMain->ShowProgress("", std::max(1, std::min(99, (int)(((double)file.tellg() / (double)nFilesize) * 100))));
        std::string line;
        std::getline(file, line);
        if (line.empty() || line[0] == '#')
            continue;
        std::vector<std::string> vstr;
        boost::split(vstr, line, boost::is_any_of(" "));
        if (vstr.size() < 2)
            continue;
        CBitcoinSecret vchSecret;
        if (!vchSecret.SetString(vstr[0]))
            continue;
        CKey key = vchSecret.GetKey();
        CPubKey pubkey = key.GetPubKey();
        assert(key.VerifyPubKey(pubkey));
        CKeyID keyid = pubkey.GetID();
        if (pwalletMain->HaveKey(keyid)) {
            LogPrintf("Skipping import of %s (key already present)\n", CBitcoinAddress(keyid).ToString());
            continue;
        }