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.

std::set<uint256> GetConflicts(const uint256& txid) const;
    
    bool HasWalletSpend(const uint256& txid) const;
    
    void Flush(bool shutdown=false);
    
    static bool Verify();
    
    /** 
     * Address book entry changed.
     * @note called with lock cs_wallet held.
     */
    boost::signals2::signal<void (CWallet *wallet, const CTxDestination
            &address, const std::string &label, bool isMine,
            const std::string &purpose,
            ChangeType status)> NotifyAddressBookChanged;
    /** 
     * Wallet transaction added, removed or updated.
     * @note called with lock cs_wallet held.
     */
    boost::signals2::signal<void (CWallet *wallet, const uint256 &hashTx,
            ChangeType status)> NotifyTransactionChanged;
    
    boost::signals2::signal<void (const std::string &title, int nProgress)> ShowProgress;
    
    boost::signals2::signal<void (bool fHaveWatchOnly)> NotifyWatchonlyChanged;
    
    bool GetBroadcastTransactions() const { return fBroadcastTransactions; }