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.

int64_t nTime = DecodeDumpTime(vstr[1]);
        std::string strLabel;
        bool fLabel = true;
        for (unsigned int nStr = 2; nStr < vstr.size(); nStr++) {
            if (boost::algorithm::starts_with(vstr[nStr], "#"))
                break;
            if (vstr[nStr] == "change=1")
                fLabel = false;
            if (vstr[nStr] == "reserve=1")
                fLabel = false;
            if (boost::algorithm::starts_with(vstr[nStr], "label=")) {
                strLabel = DecodeDumpString(vstr[nStr].substr(6));
                fLabel = true;
            }
        }
        LogPrintf("Importing %s...\n", CBitcoinAddress(keyid).ToString());
        if (!pwalletMain->AddKeyPubKey(key, pubkey)) {
            fGood = false;
            continue;
        }
        pwalletMain->mapKeyMetadata[keyid].nCreateTime = nTime;
        if (fLabel)
            pwalletMain->SetAddressBook(keyid, strLabel, "receive");
        nTimeBegin = std::min(nTimeBegin, nTime);
    }
    file.close();
    pwalletMain->ShowProgress("", 100); 
    CBlockIndex *pindex = chainActive.Tip();
    while (pindex && pindex->pprev && pindex->GetBlockTime() > nTimeBegin - 7200)
        pindex = pindex->pprev;