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.

{
        fCreditCached = false;
        fAvailableCreditCached = false;
        fImmatureCreditCached = false;
        fWatchDebitCached = false;
        fWatchCreditCached = false;
        fAvailableWatchCreditCached = false;
        fImmatureWatchCreditCached = false;
        fDebitCached = false;
        fChangeCached = false;
    }
    void BindWallet(CWallet *pwalletIn)
    {
        pwallet = pwalletIn;
        MarkDirty();
    }
    
    CAmount GetDebit(const isminefilter& filter) const;
    CAmount GetCredit(const isminefilter& filter) const;
    CAmount GetImmatureCredit(bool fUseCache=true) const;
    CAmount GetAvailableCredit(bool fUseCache=true) const;
    CAmount GetImmatureWatchOnlyCredit(const bool& fUseCache=true) const;
    CAmount GetAvailableWatchOnlyCredit(const bool& fUseCache=true) const;
    CAmount GetChange() const;
    void GetAmounts(std::list<COutputEntry>& listReceived,
                    std::list<COutputEntry>& listSent, CAmount& nFee, std::string& strSentAccount, const isminefilter& filter) const;
    void GetAccountAmounts(const std::string& strAccount, CAmount& nReceived,
                           CAmount& nSent, CAmount& nFee, const isminefilter& filter) const;
    bool IsFromMe(const isminefilter& filter) const