Unify retention index type

[matejpavlovic]

Having a separate type for the retention index for each module
seems like over-engineering, given that they are all used
exactly the same way.
I used the IDE to automatically rename everything.

[xosmig]

Eventually, it would be nice to get rid of the types and events packages or only keep really universal things there, like ModuleID and NodeID. In other words, the core should not know anything about what modules there are or how they are implemented.

That being said, since we are probably not going to get rid of types any time soon, I don't mind this PR being merged.

[xosmig]

If there are some repetitive patterns that we want to reuse across modules, we could have a moduleutils package.

[matejpavlovic]

Eventually, it would be nice to get rid of the types and events packages or only keep really universal things there, like ModuleID and NodeID.

Yes I absolutely agree. Things like PBFTViewNr have nothing to do there. It's a relict from times where Mir was not as general as it is now.

If there are some repetitive patterns that we want to reuse across modules, we could have a moduleutils package.

Yes that is definitely a good direction. I'm creating an issue so I don't forget to clean this up.