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.

return false;
                }
                for (const auto& pcoin : setCoins)
                {
                    CAmount nCredit = pcoin.first->tx->vout[pcoin.second].nValue;
                    
                    
                    
                    
                    int age = pcoin.first->GetDepthInMainChain();
                    assert(age >= 0);
                    if (age != 0)
                        age += 1;
                    dPriority += (double)nCredit * age;
                }
                const CAmount nChange = nValueIn - nValueToSelect;
                if (nChange > 0)
                {
                    
                    
                    
                    CScript scriptChange;
                    
                    if (coinControl && !boost::get<CNoDestination>(&coinControl->destChange))
                        scriptChange = GetScriptForDestination(coinControl->destChange);
                    
                    else
                    {