Add LifecycleHandler::pre_new_process

[bobrik]

My ultimate goal is to persist some state on reloads and graceful shutdown to allow the next generation to use that data. I want to be able to restore this state on reboots and crashes. The actual state is the list of cache keys to prewarm, so it's ok to have it a bit stale. It is not ok to have it empty.

When storing state in a file:

• Reading the state happens on startup, no support necessary.
• Writing needs to happen before the next generation of the process starts:
  • For graceful upgrades this pre_new_process() hook should do the trick.
  • For graceful shutdowns there would be some other thing outside of shellflip.
  • For crashes we live with stale data.

Passing the data in send_to_new_process doesn't feel right for this, but I might be persuadable. The reason I don't like it is because there would be a split in behavior on startup:

• If there's data from the parent, save it to persistent storage (some file).
• If it's a cold start without a parent, restore the data from persistent storage.