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.

CAmount& nSent, CAmount& nFee, const isminefilter& filter) const
{
    nReceived = nSent = nFee = 0;
    CAmount allFee;
    string strSentAccount;
    list<COutputEntry> listReceived;
    list<COutputEntry> listSent;
    GetAmounts(listReceived, listSent, allFee, strSentAccount, filter);
    if (strAccount == strSentAccount)
    {
        BOOST_FOREACH(const COutputEntry& s, listSent)
            nSent += s.amount;
        nFee = allFee;
    }
    {
        LOCK(pwallet->cs_wallet);
        BOOST_FOREACH(const COutputEntry& r, listReceived)