Show backups in terms of started at, not created at

When listing backups, use started_at time for 'Backup time'
Prevent redundant API call when using pg:info

Because of the way database add-ons are collected, you end up with a duplicate
for the `DATABASE_URL` database. This update rejects that entry, and only hits
  the API once for each database add-on.
Merge pull request #928 from catsby/tune_hp_info_calls

Prevent redundant API call when using pg:info
Merge pull request #926 from catsby/pgbackups_started_at_time

Show backups in terms of started at, not created at
Only loop through databases once

When mapping databases for API calls, only loop through the list once.
Merge pull request #933 from catsby/tune_hp_info_calls

Only loop through databases once