CA-108916: add call to V6rpc.get_current_edition API

We add this API so that when v6d returns a failure on apply_edition, xapi is able to find out the correct edition to set itself to. This fixes a "bug" (design decision, really) where 1) v6d holds a valid license, 2) xapi apply_edition with bad license-server url, 3) v6d release license, and fails to check out new license, 4) xapi remains at the current licensed edition, but v6d doesn't hold the license for that edition.

NB: this must be pulled in conjunction with the hg/trunk-ring3/v6.git/ca108916-get-current-edition branch.