Use ttl indexes as message scrubber

This patch makes use of mongodb's ttl collections as a way for cleaning
expired messages. In order to implement a "dynamic ttl" collection, the
expires field is literally hacked.

When a new message is creted, it's expire field is set to:
    now - ttl

The above makes the message last at most `TTL` seconds.

As for claims, the expires management didn't change, however, it now
uses datetime's instead of TS to be consistent with messages ttl.

Implements blueprint storage-mongodb

Change-Id: Ia1c5c2ca37a5fb3732d6cfa99ce13da7d376b02c
Merge "Use ttl indexes as message scrubber"