Update scoring parameters

[Shr1ftyy]

we need to update the scoring parameters as discussed here: https://discord.com/channels/799672011265015819/1321148494253264998/1389158777818972264

Thanks for the suggestion ser.

so we've done some internal testing.
we believe the following changes should help new reliable miners work up the ranks and replace more unreliable ones:

• scaling down the initial alpha and beta from alpha=500 and beta=1000 to something like alpha=10 and beta=20.
• increasing the immunity period from our current ~14 epochs to 20 epochs (7200 blocks, so 24 hrs).
• increasing the miners each piece we upload to for synthetic requests from 10 to 25. each miner that is challenged will usually be sent 3-4 pieces during each upload challenge.

this way we cover more area, and furthermore essentially amplify the effect each audit has on miners. This helps prop up very reliable miners, and heavily penalize unreliable ones.

We've also been considering reducing the forgetting factor lambda from 0.99 to something like 0.98 (or even lower) as well, but we're optimistic that the aforementioned changes should do the trick.