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 SetBroadcastTransactions(bool broadcast) { fBroadcastTransactions = broadcast; }
    
    bool AbandonTransaction(const uint256& hashTx);
    
    bool MarkReplaced(const uint256& originalHash, const uint256& newHash);
    
    static std::string GetWalletHelpString(bool showDebug);
    
    static CWallet* CreateWalletFromFile(const std::string walletFile);
    static bool InitLoadWallet();
    /**
     * Wallet post-init setup
     * Gives the wallet a chance to register repetitive tasks and complete post-init tasks
     */
    void postInitProcess(boost::thread_group& threadGroup);
    
    static bool ParameterInteraction();
    bool BackupWallet(const std::string& strDest);
    
    bool SetHDChain(const CHDChain& chain, bool memonly);
    const CHDChain& GetHDChain() { return hdChain; }