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.

ADD_SERIALIZE_METHODS;
    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action)
    {
        READWRITE(this->nVersion);
        READWRITE(nExternalChainCounter);
        READWRITE(masterKeyID);
    }
    void SetNull()
    {
        nVersion = CHDChain::CURRENT_VERSION;
        nExternalChainCounter = 0;
        masterKeyID.SetNull();
    }
};
class CKeyMetadata
{
public:
    static const int VERSION_BASIC=1;
    static const int VERSION_WITH_HDDATA=10;
    static const int CURRENT_VERSION=VERSION_WITH_HDDATA;
    int nVersion;
    int64_t nCreateTime; 
    std::string hdKeypath; 
    CKeyID hdMasterKeyID; 
    CKeyMetadata()