Extremely slow TXs with a lot of transfers

[jon4hz]

As mentioned in #102, TXs with a lot of transfers often fail.
However this isn't the only problem we encountered with high batch sizes.
The following graph shows the request duration of transactions with at most 10 transfers. As you can see, some of these calls took more than 1.33 min(!).

The yellow lines you see here are all from the transfer calls. According to the legend DERO.GetBlockTemplate is also yellow, however these calls usually took ~8ms.