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:
    unsigned int nKeys;
    unsigned int nCKeys;
    unsigned int nKeyMeta;
    bool fIsEncrypted;
    bool fAnyUnordered;
    int nFileVersion;
    vector<uint256> vWalletUpgrade;
    CWalletScanState() {
        nKeys = nCKeys = nKeyMeta = 0;
        fIsEncrypted = false;
        fAnyUnordered = false;
        nFileVersion = 0;
    }
};
bool
ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
             CWalletScanState &wss, string& strType, string& strErr)
{
    try {
        
        
        
        ssKey >> strType;