[refactoring] Timestamp renaming

[gasche]

I finally found a way to get rid of the confusing current_timestamp naming that I like.

The difficulty was that it was tempting to renaming current_timestamp into cover_state, and this worked well in many parts of the code (match switch_state.cover_state.head with ...), but some other parts of the code would have a conflict with two same-name parameters in scope: (cover_state : Cover_state.t Versioned.t) (previously current_timestamp) and (cover_state : Cover_state.t).

I solved this conflict by renaming cover_state : Cover_state.t Versioned.t into cover_state_repo : Cover_state.t Repo.t, which is now the new name for current_timestamp.

This can be reviewed either commit-by-commit or in bulk.

[Armael]

Yes, very good! Feel free to merge whenever.