Propose to rename callback hooks from "on_***" to "before_***" (or "after_***")

[johnnyshields]

Currently there are many hooks named on_***. Internally these map to options named before_***. It would be much clearer to users if we used the before/after nomenclature everywhere, as it would make it more obvious the exact timing of events.

We could alias and deprecate the old hook names for the time being.

  #     | DSL Method         |  Options Key            | Fork Block Location |
  #     | on_worker_boot     | :before_worker_boot     | inside, before      |
  #     | on_worker_shutdown | :before_worker_shutdown | inside, after       |
  #     | on_refork          | :before_refork          | inside              |

[dentarg]

Makes sense to me.

Not sure I understand the "Fork Block Location" column above. (I have not looked at any code related to this)

[dentarg]

Looks like #3258 aims to do this