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.

{
    CWalletDB walletdb(strWalletFile);
    return GetAccountBalance(walletdb, strAccount, nMinDepth, filter);
}
CAmount CWallet::GetAccountBalance(CWalletDB& walletdb, const std::string& strAccount, int nMinDepth, const isminefilter& filter)
{
    CAmount nBalance = 0;
    
    for (map<uint256, CWalletTx>::iterator it = mapWallet.begin(); it != mapWallet.end(); ++it)
    {
        const CWalletTx& wtx = (*it).second;
        if (!CheckFinalTx(wtx) || wtx.GetBlocksToMaturity() > 0 || wtx.GetDepthInMainChain() < 0)
            continue;
        CAmount nReceived, nSent, nFee;
        wtx.GetAccountAmounts(strAccount, nReceived, nSent, nFee, filter);
        if (nReceived != 0 && wtx.GetDepthInMainChain() >= nMinDepth)
            nBalance += nReceived;
        nBalance -= nSent + nFee;
    }
    
    nBalance += walletdb.GetAccountCreditDebit(strAccount);
    return nBalance;
}
std::set<CTxDestination> CWallet::GetAccountAddresses(const std::string& strAccount) const