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)
        throw runtime_error(
            "importwallet \"filename\"\n"
            "\nImports keys from a wallet dump file (see dumpwallet).\n"
            "\nArguments:\n"
            "1. \"filename\"    (string, required) The wallet file\n"
            "\nExamples:\n"
            "\nDump the wallet\n"
            + HelpExampleCli("dumpwallet", "\"test\"") +
            "\nImport the wallet\n"
            + HelpExampleCli("importwallet", "\"test\"") +
            "\nImport using the json rpc call\n"
            + HelpExampleRpc("importwallet", "\"test\"")
        );
    if (fPruneMode)
        throw JSONRPCError(RPC_WALLET_ERROR, "Importing wallets is disabled in pruned mode");
    LOCK2(cs_main, pwalletMain->cs_wallet);
    EnsureWalletIsUnlocked();