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.

CWalletDB walletdb(strWalletFile, "r+", false);
    std::set<uint256> todo;
    std::set<uint256> done;
    todo.insert(hashTx);
    while (!todo.empty()) {
        uint256 now = *todo.begin();
        todo.erase(now);
        done.insert(now);
        assert(mapWallet.count(now));
        CWalletTx& wtx = mapWallet[now];
        int currentconfirm = wtx.GetDepthInMainChain();
        if (conflictconfirms < currentconfirm) {
            
            
            wtx.nIndex = -1;
            wtx.hashBlock = hashBlock;
            wtx.MarkDirty();
            walletdb.WriteTx(wtx);
            
            TxSpends::const_iterator iter = mapTxSpends.lower_bound(COutPoint(now, 0));
            while (iter != mapTxSpends.end() && iter->first.hash == now) {
                 if (!done.count(iter->second)) {
                     todo.insert(iter->second);
                 }
                 iter++;
            }
            
            
            BOOST_FOREACH(const CTxIn& txin, wtx.tx->vin)