Add listing in ent client and add metrics

Ent client:
- add listing of all the blobs in a bucket with a given prefix
- move building a blob url to ent client

Metrics added:
- number of successful doozer backups after starting doozer-backup
- duration of the last doozer backup in seconds
- unix timestamp of the last backup in seconds

In order to get the last backup the newest blob from ent is listed. If there is still no backup, first successful backup is  created and then metrics are exposed. 

@soundcloud/iss @ohm  @juliusv 
Uh, copy&paste bug? This now has the same metric name as the one above. I think it should be `doozerd_backup_last_successful_backup_timestamp_seconds`?