Usage of required variables, "beginning with @"

There are a set of event variables, the ones that begin with `@` that somehow are required for Logstash. Even though this fields are required in the source code, and also as a core concept of logstash, the pipeline let the user remove them (see mutate filter or remove_field in filters). If something like this happen, see #4020 for more details, then some pipelines will complain.

I think is important we set the expectations about this variables right, I see different approaches here.
- We make this fields mandatory, and make this fields impossible to be removed.
- We move the timestamp and version out of the event hash and only use them as internal variables.
- We allow this variables to be removed.

I'm on the fence on this here, I see benefits of having all options. This makes me thing that probably the best option would be the second one, by moving the `@` variables as internals the event hash is more clean, allowing both usages more out of the box.