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.

{
        if (!coinControl.IsSelected(txin.prevout))
        {
            tx.vin.push_back(txin);
            if (lockUnspents)
            {
              LOCK2(cs_main, cs_wallet);
              LockCoin(txin.prevout);
            }
        }
    }
    
    if (keepReserveKey)
        reservekey.KeepKey();
    return true;
}
bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend, CWalletTx& wtxNew, CReserveKey& reservekey, CAmount& nFeeRet,