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 (fOnlyKeys)
        {
            CDataStream ssKey(row.first, SER_DISK, CLIENT_VERSION);
            CDataStream ssValue(row.second, SER_DISK, CLIENT_VERSION);
            string strType, strErr;
            bool fReadOK;
            {
                
                LOCK(dummyWallet.cs_wallet);
                fReadOK = ReadKeyValue(&dummyWallet, ssKey, ssValue,
                                        wss, strType, strErr);
            }
            if (!IsKeyType(strType) && strType != "hdchain")
                continue;
            if (!fReadOK)
            {
                LogPrintf("WARNING: CWalletDB::Recover skipping %s: %s\n", strType, strErr);
                continue;
            }
        }
        Dbt datKey(&row.first[0], row.first.size());
        Dbt datValue(&row.second[0], row.second.size());
        int ret2 = pdbCopy->put(ptxn, &datKey, &datValue, DB_NOOVERWRITE);
        if (ret2 > 0)
            fSuccess = false;
    }
    ptxn->commit(0);
    pdbCopy->close(0);