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 (pwallet == 0)
        return 0;
    
    if (IsCoinBase() && GetBlocksToMaturity() > 0)
        return 0;
    if (fUseCache && fAvailableWatchCreditCached)
        return nAvailableWatchCreditCached;
    CAmount nCredit = 0;
    for (unsigned int i = 0; i < tx->vout.size(); i++)
    {
        if (!pwallet->IsSpent(GetHash(), i))
        {
            const CTxOut &txout = tx->vout[i];
            nCredit += pwallet->GetCredit(txout, ISMINE_WATCH_ONLY);
            if (!MoneyRange(nCredit))
                throw std::runtime_error("CWalletTx::GetAvailableCredit() : value out of range");
        }
    }
    nAvailableWatchCreditCached = nCredit;
    fAvailableWatchCreditCached = true;
    return nCredit;
}
CAmount CWalletTx::GetChange() const