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.

else if (strType == "watchs")
        {
            CScript script;
            ssKey >> *(CScriptBase*)(&script);
            char fYes;
            ssValue >> fYes;
            if (fYes == '1')
                pwallet->LoadWatchOnly(script);
            
            
            pwallet->UpdateTimeFirstKey(1);
        }
        else if (strType == "key" || strType == "wkey")
        {
            CPubKey vchPubKey;
            ssKey >> vchPubKey;
            if (!vchPubKey.IsValid())
            {
                strErr = "Error reading wallet database: CPubKey corrupt";
                return false;
            }
            CKey key;
            CPrivKey pkey;
            uint256 hash;
            if (strType == "key")
            {
                wss.nKeys++;
                ssValue >> pkey;
            } else {
                CWalletKey wkey;