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.

}
                else
                    reservekey.ReturnKey();
                
                
                
                
                
                
                
                
                
                
                for (const auto& coin : setCoins)
                    txNew.vin.push_back(CTxIn(coin.first->GetHash(),coin.second,CScript(),
                                              std::numeric_limits<unsigned int>::max() - (fWalletRbf ? 2 : 1)));
                
                if (!DummySignTx(txNew, setCoins)) {
                    strFailReason = _("Signing transaction failed");
                    return false;
                }
                unsigned int nBytes = GetVirtualTransactionSize(txNew);
                CTransaction txNewConst(txNew);
                dPriority = txNewConst.ComputePriority(dPriority, nBytes);
                
                for (auto& vin : txNew.vin) {
                    vin.scriptSig = CScript();
                    vin.scriptWitness.SetNull();
                }