Improve spend txn handling

[AliAshrafD]

Currently, the wallet pre-empts the total balance by including all UTXOs in the transaction no matter what the actual output amount may be. In the case of wallets with multiple tiny inputs this approach results in giant transactions even when the owner is spending a fraction of the balance.

[AliAshrafD]

I made a pull request for fixing this. The idea is spending utxos with smaller amounts in the first few transactions. For this to happen we sort the unspent array first (ascending ordered by amount), then we spend the minimum required number of the topmost items as the inputs to the txn under construction. It is also possible to sort the array in reverse order (descending) using the largets outputs first, it would reduce the number of inputs for the first txns, but leaves more dusts in the database.
By the way, is this a dead project?

[AliAshrafD]

For now, I commented-out the sort part because of performance issues.

[AliAshrafD]

Just tested the wallet with this patch, it works fine, I managed to spend 1000 utxos gradually 70-80 each time. Good news, I believe.