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.

{
public:
    std::string strAccount;
    CAmount nCreditDebit;
    int64_t nTime;
    std::string strOtherAccount;
    std::string strComment;
    mapValue_t mapValue;
    int64_t nOrderPos; 
    uint64_t nEntryNo;
    CAccountingEntry()
    {
        SetNull();
    }
    void SetNull()
    {
        nCreditDebit = 0;
        nTime = 0;
        strAccount.clear();
        strOtherAccount.clear();
        strComment.clear();
        nOrderPos = -1;
        nEntryNo = 0;
    }
    ADD_SERIALIZE_METHODS;
    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {