Don't use claim creation time to calc claim's age

Mongodb's driver uses Claim's creation time to estimate claims age
instead, it should use claim's update time. Since there's no update /
creation time in mongodb's claims, the patch estimates claims age by
doing:

    delta(expiration - ttl, now)

This change fixes the issue bellow and makes mongodb's claims compliant
with Marconi's specs.

Change-Id: I72c968236ecd239e7b885b293238e2920807a57c
Closes-Bug: #1210662
Merge "Don't use claim creation time to calc claim's age"