Add after_refork hook

[Drakula2k]

Description

Adds after_refork hook to be able to re-create the resources that were removed in on_refork on the worker 0 (Mongo connections in my use case).

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.

[johnnyshields]

@nateberkopec the use case for after_refork is:

• We use before_refork to disconnect any active database sockets on worker0
• We use on_worker_boot to reconnect database/restart monitoring threads on the grandchild instance
• However, there is currently no way to reconnect on worker0; worker0 stays disconnected even after forking.

Note that after_refork triggers only on worker0 and not on the grandchilds.