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.

{
    {
        LOCK(cs_wallet);
        map<uint256, CWalletTx>::const_iterator mi = mapWallet.find(txin.prevout.hash);
        if (mi != mapWallet.end())
        {
            const CWalletTx& prev = (*mi).second;
            if (txin.prevout.n < prev.tx->vout.size())
                return IsMine(prev.tx->vout[txin.prevout.n]);
        }
    }
    return ISMINE_NO;
}
CAmount CWallet::GetDebit(const CTxIn &txin, const isminefilter& filter) const
{
    {
        LOCK(cs_wallet);
        map<uint256, CWalletTx>::const_iterator mi = mapWallet.find(txin.prevout.hash);
        if (mi != mapWallet.end())
        {
            const CWalletTx& prev = (*mi).second;
            if (txin.prevout.n < prev.tx->vout.size())
                if (IsMine(prev.tx->vout[txin.prevout.n]) & filter)
                    return prev.tx->vout[txin.prevout.n].nValue;
        }
    }
    return 0;
}
isminetype CWallet::IsMine(const CTxOut& txout) const