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.

class CReserveKey;
class CScript;
class CTxMemPool;
class CWalletTx;
enum WalletFeature
{
    FEATURE_BASE = 10500, 
    FEATURE_WALLETCRYPT = 40000, 
    FEATURE_COMPRPUBKEY = 60000, 
    FEATURE_HD = 130000, 
    FEATURE_LATEST = FEATURE_COMPRPUBKEY 
};
class CKeyPool
{
public:
    int64_t nTime;
    CPubKey vchPubKey;
    CKeyPool();
    CKeyPool(const CPubKey& vchPubKeyIn);
    ADD_SERIALIZE_METHODS;
    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {
        int nVersion = s.GetVersion();
        if (!(s.GetType() & SER_GETHASH))
            READWRITE(nVersion);
        READWRITE(nTime);
        READWRITE(vchPubKey);
    }
};
class CAddressBookData