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.

{
    {
        AssertLockHeld(cs_wallet);
        if (posInBlock != -1) {
            BOOST_FOREACH(const CTxIn& txin, tx.vin) {
                std::pair<TxSpends::const_iterator, TxSpends::const_iterator> range = mapTxSpends.equal_range(txin.prevout);
                while (range.first != range.second) {
                    if (range.first->second != tx.GetHash()) {
                        LogPrintf("Transaction %s (in block %s) conflicts with wallet transaction %s (both spend %s:%i)\n", tx.GetHash().ToString(), pIndex->GetBlockHash().ToString(), range.first->second.ToString(), range.first->first.hash.ToString(), range.first->first.n);
                        MarkConflicted(pIndex->GetBlockHash(), range.first->second);
                    }
                    range.first++;
                }
            }
        }
        bool fExisted = mapWallet.count(tx.GetHash()) != 0;
        if (fExisted && !fUpdate) return false;
        if (fExisted || IsMine(tx) || IsFromMe(tx))
        {
            CWalletTx wtx(this, MakeTransactionRef(tx));
            
            if (posInBlock != -1)
                wtx.SetMerkleBranch(pIndex, posInBlock);
            return AddToWallet(wtx, false);
        }
    }
    return false;
}
bool CWallet::AbandonTransaction(const uint256& hashTx)