Refactor "mining" / PoW references in base classes

[fselmo]

What is wrong?

We have some references around mining built into the code base. Though the execution logic may be updated for the merge, we should go back through and re-organize a lot of the code, especially from base classes, to make more sense for the PoS era. Most of these changes will revolve around "mining" but there may be other changes that don't make sense for the base classes to bubble up as well.

i.e. Things like mine_block() in the base vm class

Perhaps this is a good time to refactor some code along the way.