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.

{
        delete pwalletdbEncryption;
        pwalletdbEncryption = NULL;
    }
    void SetNull()
    {
        nWalletVersion = FEATURE_BASE;
        nWalletMaxVersion = FEATURE_BASE;
        fFileBacked = false;
        nMasterKeyMaxID = 0;
        pwalletdbEncryption = NULL;
        nOrderPosNext = 0;
        nNextResend = 0;
        nLastResend = 0;
        nTimeFirstKey = 0;
        fBroadcastTransactions = false;
    }
    std::map<uint256, CWalletTx> mapWallet;
    std::list<CAccountingEntry> laccentries;
    typedef std::pair<CWalletTx*, CAccountingEntry*> TxPair;
    typedef std::multimap<int64_t, TxPair > TxItems;
    TxItems wtxOrdered;
    int64_t nOrderPosNext;
    std::map<uint256, int> mapRequestCount;
    std::map<CTxDestination, CAddressBookData> mapAddressBook;
    CPubKey vchDefaultKey;
    std::set<COutPoint> setLockedCoins;
    const CWalletTx* GetWalletTx(const uint256& hash) const;
    
    bool CanSupportFeature(enum WalletFeature wf) { AssertLockHeld(cs_wallet); return nWalletMaxVersion >= wf; }