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.

{
    if (!EnsureWalletIsAvailable(request.fHelp))
        return NullUniValue;
    
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 3)
        throw runtime_error(
            "importprivkey \"bitcoinprivkey\" ( \"label\" ) ( rescan )\n"
            "\nAdds a private key (as returned by dumpprivkey) to your wallet.\n"
            "\nArguments:\n"
            "1. \"bitcoinprivkey\"   (string, required) The private key (see dumpprivkey)\n"
            "2. \"label\"            (string, optional, default=\"\") An optional label\n"
            "3. rescan               (boolean, optional, default=true) Rescan the wallet for transactions\n"
            "\nNote: This call can take minutes to complete if rescan is true.\n"
            "\nExamples:\n"
            "\nDump a private key\n"
            + HelpExampleCli("dumpprivkey", "\"myaddress\"") +
            "\nImport the private key with rescan\n"
            + HelpExampleCli("importprivkey", "\"mykey\"") +
            "\nImport using a label and without rescan\n"
            + HelpExampleCli("importprivkey", "\"mykey\" \"testing\" false") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("importprivkey", "\"mykey\", \"testing\", false")
        );
    LOCK2(cs_main, pwalletMain->cs_wallet);
    EnsureWalletIsUnlocked();
    string strSecret = request.params[0].get_str();
    string strLabel = "";
    if (request.params.size() > 1)