Signing after UPDATE fixed

Fixed changeset signing after UPDATE.

Was ignoring RRSIGs related to a RRSet that was completely removed. This was a relict of old code when RRSIGs were connected to their RRSets.

Also simplified the function (and function `sign_rr_should_be_signed()`) and did some minor refactoring and polishing. Please try to check if function `sign_changeset_wrap()` is equivalent to its previous state.