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.

int64_t nLastResend;
    bool fBroadcastTransactions;
    /**
     * Used to keep track of spent outpoints, and
     * detect and report conflicts (double-spends or
     * mutated transactions where the mutant gets mined).
     */
    typedef std::multimap<COutPoint, uint256> TxSpends;
    TxSpends mapTxSpends;
    void AddToSpends(const COutPoint& outpoint, const uint256& wtxid);
    void AddToSpends(const uint256& wtxid);
    
    void MarkConflicted(const uint256& hashBlock, const uint256& hashTx);
    void SyncMetaData(std::pair<TxSpends::iterator, TxSpends::iterator>);
    
    CHDChain hdChain;
    bool fFileBacked;
    std::set<int64_t> setKeyPool;
    int64_t nTimeFirstKey;
public:
    /*
     * Main wallet lock.
     * This lock protects all the fields added by CWallet
     *   except for:
     *      fFileBacked (immutable after instantiation)
     *      strWalletFile (immutable after instantiation)
     */
    mutable CCriticalSection cs_wallet;
    const std::string strWalletFile;
    void LoadKeyPool(int nIndex, const CKeyPool &keypool)