Check if a message is validated at compile time

[dmdashenkov]

SpineEventEngine/base#409 introduces @Validated and @NotValidated annotations. Basing on them, we may add create a static analyzer (e.g. an ErrorProne check) and make sure that users as well as we, do not use not validated messages.

The state of a message of being validated or not should be treated similarly to an object's nullability. We may pass not validated messages around, but not use their values.