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.

{
    
    vector<uint256> vTxHash;
    vector<CWalletTx> vWtx;
    DBErrors err = FindWalletTx(pwallet, vTxHash, vWtx);
    if (err != DB_LOAD_OK) {
        return err;
    }
    std::sort(vTxHash.begin(), vTxHash.end());
    std::sort(vTxHashIn.begin(), vTxHashIn.end());
    
    bool delerror = false;
    vector<uint256>::iterator it = vTxHashIn.begin();
    BOOST_FOREACH (uint256 hash, vTxHash) {