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.

{
        ReturnKey();
    }
    void ReturnKey();
    bool GetReservedKey(CPubKey &pubkey);
    void KeepKey();
    void KeepScript() { KeepKey(); }
};
/** 
 * Account information.
 * Stored in wallet with key "acc"+string account name.
 */
class CAccount
{
public:
    CPubKey vchPubKey;
    CAccount()
    {
        SetNull();
    }
    void SetNull()
    {
        vchPubKey = CPubKey();
    }
    ADD_SERIALIZE_METHODS;
    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {