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.

strErr = "Error reading wallet database: CPrivKey corrupt";
                return false;
            }
            if (!pwallet->LoadKey(key, vchPubKey))
            {
                strErr = "Error reading wallet database: LoadKey failed";
                return false;
            }
        }
        else if (strType == "mkey")
        {
            unsigned int nID;
            ssKey >> nID;
            CMasterKey kMasterKey;
            ssValue >> kMasterKey;
            if(pwallet->mapMasterKeys.count(nID) != 0)
            {
                strErr = strprintf("Error reading wallet database: duplicate CMasterKey id %u", nID);
                return false;
            }
            pwallet->mapMasterKeys[nID] = kMasterKey;
            if (pwallet->nMasterKeyMaxID < nID)
                pwallet->nMasterKeyMaxID = nID;
        }
        else if (strType == "ckey")
        {
            CPubKey vchPubKey;
            ssKey >> vchPubKey;
            if (!vchPubKey.IsValid())
            {