Small optimizations

[nazar-pc]

I was double-checking claims from some users that plotting in feb-19 release is slower than feb-05 and found no proof for this on my machine with 13900K.

I did update plotting benchmark to be more representative of how farmer does plotting and updated blst to the version from supranational/blst#203 (except last commit that bumps version) that I pushed to our fork: https://github.com/subspace/blst/tree/subspace-v2

Code contributor checklist:

• I have read, understood and followed contributing guide