Refactor or rewrite `block.Manager`

[tzdybal]

Code in block.Manager is quite complicated, and it's obvious it was refactored multiple times. It's not very straight-forward to read and maintain.

As the changes related to Execution API changes the flow significantly, IMHO it's worth considering rewriting Manager to simplify code and make it more readable.

We have well defined APIs (DA, Sequencing, Execution) so we can create expressive and readable code from scratch.