Use a new client for the download call.
Add a comment about why we are using a new API client for download.
Add a todo comment.
Merge pull request #1275 from ericsnowcurrently/backups-fix-api-conn

Use a new client for the backups download call.

Apparently an API client is good for only one connection.

(See http://juju-ci.vapour.ws:8080/job/functional-ha-backup-restore/1190/console.)

(Review request: http://reviews.vapour.ws/r/590/)