Added Mixin-related filters on DB queries

As a pre-requisite for being able to query the database for objects
that are expired, I need a way to express date comparison on the
'deleted_at' field which is common for every Watcher object. As they
are coming from mixins, I decided to implement these filters with a
syntax borrowed from the Django ORM where the field is suffixed by the
comparison operator you want to apply:

- The '__lt' suffix stands for 'less than'
- The '__lte' suffix stands for 'less than or equal to'
- The '__gt' suffix stands for 'greater than'
- The '__gte' suffix stands for 'greater than or equal to'
- The '__eq' suffix stands for 'equal to'

I also added a 'uuid' filter to later on be able to filter by uuid.

Partially Implements: blueprint db-purge-engine

Change-Id: I763f330c1b8ea8395990d2276b71e87f5b3f3ddc

test-requirements.txt

@@ -5,6 +5,7 @@
 coverage>=3.6 # Apache-2.0
 discover # BSD
 doc8 # Apache-2.0
+freezegun # Apache-2.0
 hacking<0.11,>=0.10.2
 mock>=1.2 # BSD
 oslotest>=1.10.0 # Apache-2.0