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 (GetBoolArg("-disablewallet", DEFAULT_DISABLE_WALLET)) {
        pwalletMain = NULL;
        LogPrintf("Wallet disabled!\n");
        return true;
    }
    std::string walletFile = GetArg("-wallet", DEFAULT_WALLET_DAT);
    CWallet * const pwallet = CreateWalletFromFile(walletFile);
    if (!pwallet) {
        return false;
    }
    pwalletMain = pwallet;
    return true;
}
std::atomic<bool> CWallet::fFlushThreadRunning(false);
void CWallet::postInitProcess(boost::thread_group& threadGroup)