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: CPubKey corrupt";
                return false;
            }
            vector<unsigned char> vchPrivKey;
            ssValue >> vchPrivKey;
            wss.nCKeys++;
            if (!pwallet->LoadCryptedKey(vchPubKey, vchPrivKey))
            {
                strErr = "Error reading wallet database: LoadCryptedKey failed";
                return false;
            }
            wss.fIsEncrypted = true;
        }
        else if (strType == "keymeta")
        {
            CPubKey vchPubKey;
            ssKey >> vchPubKey;
            CKeyMetadata keyMeta;
            ssValue >> keyMeta;
            wss.nKeyMeta++;
            pwallet->LoadKeyMetadata(vchPubKey, keyMeta);
        }
        else if (strType == "defaultkey")
        {
            ssValue >> pwallet->vchDefaultKey;
        }
        else if (strType == "pool")
        {
            int64_t nIndex;
            ssKey >> nIndex;