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 (nOrderPos >= nOffsetStart)
                    ++nOrderPosOff;
            }
            nOrderPos += nOrderPosOff;
            nOrderPosNext = std::max(nOrderPosNext, nOrderPos + 1);
            if (!nOrderPosOff)
                continue;
            
            if (pwtx)
            {
                if (!walletdb.WriteTx(*pwtx))
                    return DB_LOAD_FAIL;
            }
            else
                if (!walletdb.WriteAccountingEntry(pacentry->nEntryNo, *pacentry))
                    return DB_LOAD_FAIL;
        }
    }
    walletdb.WriteOrderPosNext(nOrderPosNext);
    return DB_LOAD_OK;
}
int64_t CWallet::IncOrderPosNext(CWalletDB *pwalletdb)