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.

void UpdateTimeFirstKey(int64_t nCreateTime);
    
    bool AddCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret);
    
    bool LoadCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret);
    bool AddCScript(const CScript& redeemScript);
    bool LoadCScript(const CScript& redeemScript);
    
    bool AddDestData(const CTxDestination &dest, const std::string &key, const std::string &value);
    
    bool EraseDestData(const CTxDestination &dest, const std::string &key);
    
    bool LoadDestData(const CTxDestination &dest, const std::string &key, const std::string &value);
    
    bool GetDestData(const CTxDestination &dest, const std::string &key, std::string *value) const;
    
    bool AddWatchOnly(const CScript &dest);
    bool RemoveWatchOnly(const CScript &dest);
    
    bool LoadWatchOnly(const CScript &dest);
    bool Unlock(const SecureString& strWalletPassphrase);
    bool ChangeWalletPassphrase(const SecureString& strOldWalletPassphrase, const SecureString& strNewWalletPassphrase);
    bool EncryptWallet(const SecureString& strWalletPassphrase);
    void GetKeyBirthTimes(std::map<CKeyID, int64_t> &mapKeyBirth) const;
    /** 
     * Increment the next transaction order id
     * @return next transaction order id
     */