fix: fine tune RandomX algo minimum difficulty

[ksrichard]

Description

Fine tuned RandomX calculated difficulty to let anybody with a normal desktop could submit shares.

Motivation and Context

On a normal desktop it was not really possible to get a share using RandomX algorithm because of the difficulty we used (minimum difficulty is used from constants for all network if the calculated one is too low).

How Has This Been Tested?

Running this version of p2pool with Tari Universe.

Test device: Macbook Pro, M1 Pro CPU
Results:

• using ECO mode (around 820 Hashes/sec) I was able to submit around 1 block/3 minutes
• using Ludicrous mode (around 2000 hashes/sec) produces around 1 block/1 minute (sometimes 2 blocks/1 minute)

What process can a PR reviewer use to test or verify this change?

See test.

Breaking Changes

• None
• Requires data directory on base node to be deleted
• Requires hard fork
• Other - Please specify - P2pool needs to be updated on all universe instances