Emmystar: small updates in the parameters of the delay function

There are two changes with respect to the initial proposal:

• the threshold for the "fast path" blocks has been increased from 50% of the total endorsement power per block to 60%,
• the de = delay_per_missing_endorsements constant has been decreased from 8 to 4.

The first change is meant to mitigate an attack where the baker at priority 0 would produce two blocks at the same time, in which case each block would receive a bit less than half of the endorsements from the other bakers, while the malicious baker would endorse both blocks, making both blocks satisfy the requirement on the number of endorsements. In such a case there would be a fork (of length 1). Increasing the threshold makes this attack significantly harder. However, increasing the threshold too much has however the consequence that selfish baking starts to become profitable. We find that a threshold of 60% offers a good trade-off.

The second change is meant to increase the number of endorsement slots that make the difference between being able to inject a "slow path" block at priority 0 and a block at priority 1. This difference is given by dp/de. For de=8, this difference was of 5 endorsements slots, while with de=4 the difference is of 10 endorsements slots. A smaller de also reduces the block times in case there very few endorsements to more reasonable values.