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 
Any special reason to return a value type here? If you return `*ent.ResponseFile` as a pointer, you could just always return `nil` in case of errors instead of creating an empty `ResponseFile`. Same goes for the other functions with that return value.