v2.3.1: reduce dependencies, move dependencies to indirect

[PietrH]

This is quite an easy one, after #317 a number of our dependencies become indirect via etnservice and while there is no real harm in keeping them here, it'll be easier to reduce dependencies if they are only declared on etnservice.

I tried using lintr to detect them, but it seems to be only sensitive to direct calls to library(), another tactic could be to search for namespace declaration ::, or (and this is tempting), removing all dependencies, running R CMD CHECK and looking at the errors.

[PietrH]

It would be great if we could drop odbc as a dependency, it's still Suggested as it's used in tests to check to see if there is a local connection to the etn database.

[PietrH]

We could even make etnservice an optional dependency, making etn itself much easier to install.