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

marconi/storage/mongodb/claims.py

@@ -72,8 +72,6 @@ class ClaimController(storage.ClaimBase):
         except ValueError:
             raise exceptions.ClaimDoesNotExist()
 
-        age = timeutils.delta_seconds(utils.oid_utc(cid), now)
-
         def messages(msg_iter):
             msg = next(msg_iter)
             yield msg.pop('claim')
@@ -91,6 +89,10 @@ class ClaimController(storage.ClaimBase):
             msgs = messages(msg_ctrl.claimed(queue, cid, now,
                                              project=project))
             claim = next(msgs)
+
+            update_time = claim['e'] - datetime.timedelta(seconds=claim['t'])
+            age = timeutils.delta_seconds(update_time, now)
+
             claim = {
                 'age': int(age),
                 'ttl': claim.pop('t'),

marconi/tests/transport/wsgi/test_claims.py

@@ -21,6 +21,7 @@ import pymongo
 import falcon
 
 from marconi.common import config
+from marconi.openstack.common import timeutils
 from marconi.tests.transport.wsgi import base
 
 
@@ -154,15 +155,19 @@ class ClaimsBaseTest(base.TestBase):
 
         # Update the claim
         new_claim_ttl = '{"ttl": 60}'
+        creation = timeutils.utcnow()
         self.simulate_patch(claim_href, self.project_id, body=new_claim_ttl)
         self.assertEquals(self.srmock.status, falcon.HTTP_204)
 
         # Get the claimed messages (again)
         body = self.simulate_get(claim_href, self.project_id)
+        query = timeutils.utcnow()
         claim = json.loads(body[0])
         message_href, params = claim['messages'][0]['href'].split('?')
 
         self.assertEquals(claim['ttl'], 60)
+        estimated_age = timeutils.delta_seconds(creation, query)
+        self.assertTrue(estimated_age > claim['age'])
 
         # Delete the claim
         self.simulate_delete(claim['href'], 'bad_id')