Move definition of block forging loop back to the ouroboros-consensus package

Our high-level idea is:

- `ouroboros-consensus` defines general abstractions and components

- `ouroboros-consensus` assembles those components and integrates them with the Networking team's diffusion layer

The block forging loop is a component, and should therefore be defined in `ouroboros-consensus`.

(Maybe someday we could abstract over the diffusion layer and therefore move the assembling of the components into `ouroboros-consensus` as well, but today it seems natural to integrate with the diffusion layer and assemble the components all in one place.)