Add explicit Crystal::EventLoop#reopened(FileDescriptor) hook

[ysbaddaden]

We don't want to close the file descriptor after we reopened it. Instead, some event loops want to resume the pending operations on the old file descriptor, so they can be restarted on the new file descriptor and at worst wait on the new one.

The proposed #reopened(FileDescriptor) method explicits this difference in intended behavior.

It doesn't change anything for the current event loops, but io_uring for example can close asynchronously so we must make the distinction, because we definitely don't want to close after reopen 😅

Another pull request will propose to move the actual close to the EventLoop so the #close(io) method will do what it's supposed to do (close the FileDescriptor / Socket).

Extracted from #15634

[Sija]

As it's a hook I'd suggest naming it on_reopen to make it more clear and conventional.

[ysbaddaden]

I tried #after_reopen to be explicit that it was called after reopen, but that didn't read better than #reopened that also conveys this feeling. But #on_reopen would lost that information: does it happen before, after, during?

[Sija]

on_reopened then?

[straight-shoota]

I think the method name is fine. Calling this methods signals to the event loop that a file descriptor was reopened.