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.

ssValue >> wkey;
                pkey = wkey.vchPrivKey;
            }
            
            
            
            
            
            try
            {
                ssValue >> hash;
            }
            catch (...) {}
            bool fSkipCheck = false;
            if (!hash.IsNull())
            {
                
                std::vector<unsigned char> vchKey;
                vchKey.reserve(vchPubKey.size() + pkey.size());
                vchKey.insert(vchKey.end(), vchPubKey.begin(), vchPubKey.end());
                vchKey.insert(vchKey.end(), pkey.begin(), pkey.end());
                if (Hash(vchKey.begin(), vchKey.end()) != hash)
                {
                    strErr = "Error reading wallet database: CPubKey/CPrivKey corrupt";
                    return false;
                }
                fSkipCheck = true;
            }
            if (!key.Load(pkey, vchPubKey, fSkipCheck))
            {