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.

{
private:
    const CWallet* pwallet;
public:
    mapValue_t mapValue;
    std::vector<std::pair<std::string, std::string> > vOrderForm;
    unsigned int fTimeReceivedIsTxTime;
    unsigned int nTimeReceived; 
    unsigned int nTimeSmart;
    /**
     * From me flag is set to 1 for transactions that were created by the wallet
     * on this bitcoin node, and set to 0 for transactions that were created
     * externally and came in through the network or sendrawtransaction RPC.
     */
    char fFromMe;
    std::string strFromAccount;
    int64_t nOrderPos; 
    
    mutable bool fDebitCached;
    mutable bool fCreditCached;