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.

bool DelAddressBook(const CTxDestination& address);
    void UpdatedTransaction(const uint256 &hashTx);
    void Inventory(const uint256 &hash)
    {
        {
            LOCK(cs_wallet);
            std::map<uint256, int>::iterator mi = mapRequestCount.find(hash);
            if (mi != mapRequestCount.end())
                (*mi).second++;
        }
    }
    void GetScriptForMining(boost::shared_ptr<CReserveScript> &script);
    void ResetRequestCount(const uint256 &hash)
    {
        LOCK(cs_wallet);
        mapRequestCount[hash] = 0;
    };
    
    unsigned int GetKeyPoolSize()
    {
        AssertLockHeld(cs_wallet); 
        return setKeyPool.size();
    }
    bool SetDefaultKey(const CPubKey &vchPubKey);
    
    bool SetMinVersion(enum WalletFeature, CWalletDB* pwalletdbIn = NULL, bool fExplicit = false);
    
    bool SetMaxVersion(int nVersion);
    
    int GetVersion() { LOCK(cs_wallet); return nWalletVersion; }