-rescan is very slow

[dooglus]

I was seeing a -rescan take many hours, when previously it was faster.

I traced the problem to this code in wallet.cpp:

        BOOST_FOREACH(const CTxOut& txout, tx.vout)
        {
            if (IsMine(txout))
            {
                wtx.MarkUnspent(&txout - &tx.vout[0]);
                wtx.WriteToDisk();
                NotifyTransactionChanged(this, hash, CT_UPDATED);
            }
        }

it is writing the same potentially large transaction to the wallet database over and over if multiple outputs belong to us.

http://khashier.com/tx/206455f7636bbef3e1161f5d2a595a8787a58056adf2966039fcafdae7fb3d17#i0 was particularly slow - the above loop takes almost 3 minutes to run on that one transaction as it writes the same tx to disk ~1000 times.

How about replacing with something like this?

        bool fMine = false;
        BOOST_FOREACH(const CTxOut& txout, tx.vout)
        {
            if (IsMine(txout))
            {
                wtx.MarkUnspent(&txout - &tx.vout[0]);
                fMine = true;
            }
        }

        if (fMine) {
            wtx.WriteToDisk();
            NotifyTransactionChanged(this, hash, CT_UPDATED);
        }

Is it safe?

[dooglus]

Fixed in #157.