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.

int64_t GetOldestKeyPoolTime();
    void GetAllReserveKeys(std::set<CKeyID>& setAddress) const;
    std::set< std::set<CTxDestination> > GetAddressGroupings();
    std::map<CTxDestination, CAmount> GetAddressBalances();
    CAmount GetAccountBalance(const std::string& strAccount, int nMinDepth, const isminefilter& filter);
    CAmount GetAccountBalance(CWalletDB& walletdb, const std::string& strAccount, int nMinDepth, const isminefilter& filter);
    std::set<CTxDestination> GetAccountAddresses(const std::string& strAccount) const;
    isminetype IsMine(const CTxIn& txin) const;
    /**
     * Returns amount of debit if the input matches the
     * filter, otherwise returns 0
     */
    CAmount GetDebit(const CTxIn& txin, const isminefilter& filter) const;
    isminetype IsMine(const CTxOut& txout) const;
    CAmount GetCredit(const CTxOut& txout, const isminefilter& filter) const;
    bool IsChange(const CTxOut& txout) const;
    CAmount GetChange(const CTxOut& txout) const;
    bool IsMine(const CTransaction& tx) const;
    
    bool IsFromMe(const CTransaction& tx) const;
    CAmount GetDebit(const CTransaction& tx, const isminefilter& filter) const;
    
    bool IsAllFromMe(const CTransaction& tx, const isminefilter& filter) const;
    CAmount GetCredit(const CTransaction& tx, const isminefilter& filter) const;
    CAmount GetChange(const CTransaction& tx) const;
    void SetBestChain(const CBlockLocator& loc);
    DBErrors LoadWallet(bool& fFirstRunRet);
    DBErrors ZapWalletTx(std::vector<CWalletTx>& vWtx);
    DBErrors ZapSelectTx(std::vector<uint256>& vHashIn, std::vector<uint256>& vHashOut);
    bool SetAddressBook(const CTxDestination& address, const std::string& strName, const std::string& purpose);