adjust veryfying-workers to 3/4 of the proving threads IF these are set.

[pigmej]

The community has a proposal that we should set the verifying-workers to 3/4 or all of the proving threads IF these are set.

And we definitely should decrease defaults from 3/4 threads to 1/4 instead.

[reythia]

This would remove the unexpected 'side effect' of one node finishing proof then competing for resources against nodes still generating proof.

This could be a signifcant contributor to why users - including myself - correlate ATX processing with being unable to finish proof in time.

[pigmej]

For now, we defaulted to 1/2 cores and added a dedicated section in smesher-guide: https://github.com/spacemeshos/wiki/wiki/Smesher-Guide#performance-fine-tuning-of-a-node