Follow-up Events

[Matej Pavlovic]

Use the newly introduced FollowUp and FollowUps methods instead of directly accessing the Event's Next field.
This task corresponds to the following TODO:

mirbft/pkg/pb/eventpb/methods.go

Line 3 in ba531de

 // TODO: Use the FollowUp function instead of direct access to the Event's Next field. 

This task involves looking up all direct accesses to the Next field and replacing them by the appropriate calls to FollowUp and FollowUps